Refrigeration apparatus

ABSTRACT

A refrigeration apparatus performs heat exchange on a water tube system having a water inlet tube leading exterior water to a water branching point, first and second branching water tubes extending from the water branching point, and a water outlet tube leading to the exterior from a convergent point of the first and second branching water tubes. Active refrigerant is in a supercritical state in at least part of a refrigeration cycle. The refrigeration apparatus includes a main expansion mechanism connected to an evaporator, first and second compression elements connected by a first refrigerant tube, a first heat exchanger exchanging heat between the first refrigerant tube and the first branching water tubes, second refrigerant tubes connecting the second compression element and the main expansion mechanism, and a second heat exchanger in which the second refrigerant tubes exchange heat with the second branching water tubes and does not exchange heat with the water inlet tube.

TECHNICAL FIELD

The present invention relates to a refrigeration apparatus, andparticularly relates to a refrigeration apparatus which performs amulti-stage compression-type refrigeration cycle using refrigerant thatoperates including the process of a supercritical state.

BACKGROUND ART

In conventional practice, one example of a refrigeration apparatus whichperforms a multi-stage compression-type refrigeration cycle usingrefrigerant that operates in a supercritical range is anair-conditioning apparatus which performs a two-stage compression-typerefrigeration cycle using carbon dioxide as a refrigerant, such as theapparatus disclosed in Patent Literature 1 (Japanese Laid-open PatentApplication No. 2007-232263).

An example of an apparatus that uses such a two-stage compression-typerefrigeration cycle as a water heater is a water heater such as the onedisclosed in Patent Literature 2 (Japanese Laid-open Patent ApplicationNo. 2002-106988), for example. In this water heater, a conventionaltechnique is used for improving compression efficiency by using anintercooler to cool refrigerant heading from a low-stage compressionelement to a high-stage compression element. Not only is water for a hotwater supply heated in a gas cooler, but some of this heated water isbranched off, part is successively led to and heated in the gas coolerwhile the other part is led to and heated in an intercooler, and hotwater for a hot water supply is obtained. Thus, the intercooler can beused as a heater of hot water and also as a cooler of refrigerant drawninto the high-stage compression element, and energy efficiency can beimproved.

SUMMARY OF INVENTION Technical Problem

In the water heater described above, water flowing into the intercoolerhas already been heated when passing through the gas cooler, and is warmwater having a somewhat high temperature. Therefore, there could becases in which the temperature of the warm water that has passed throughthe gas cooler and been heated is higher than the temperature of therefrigerant passing through the intercooler, for example. In such cases,not only is it not possible to heat the water in the intercooler, but itis also not possible to cool the refrigerant drawn into the high-stagecompression element, and it is therefore not possible to improvecompression efficiency.

An object of the present invention is to provide a refrigerationapparatus which uses refrigerant that operates including the process ofa supercritical state, wherein it is possible to more reliably improvecompression efficiency and make the heating of water for a hot watersupply more efficient.

Solution to Problem

A refrigeration apparatus according to a first aspect of the presentinvention is refrigeration apparatus which performs heat exchange on awater tube system having a water inlet tube for leading water suppliedfrom the exterior to a water branching point, first branching watertubes and second branching water tubes extending from the waterbranching point, and a water outlet tube leading out to the exteriorfrom a convergent point where the first branching water tubes and thesecond branching water tubes converge, wherein the active refrigerant isin a supercritical state in at least part of the refrigeration cycle;the refrigeration apparatus comprising a main expansion mechanism, anevaporator, a first compression element, a second compression element, afirst refrigerant tube, a first heat exchanger, second refrigeranttubes, and a second heat exchanger. The main expansion mechanismdepressurizes the refrigerant. The evaporator is connected with the mainexpansion mechanism and the evaporator evaporates the refrigerant. Thefirst compression element draws in refrigerant that has passed throughthe evaporator and compresses and discharges the refrigerant. The secondcompression element draws in the refrigerant discharged from the firstcompression element and further compresses and discharges therefrigerant. The first refrigerant tube draws the refrigerant dischargedfrom the first compression element into the second compression element.The first heat exchanger performs heat exchange between the refrigerantpassing through the first refrigerant tube and the water flowing throughthe first branching water tubes. The second refrigerant tubes connectthe discharge side of the second compression element and the mainexpansion mechanism. The second heat exchanger subjects the refrigerantpassing through the second refrigerant tubes to heat exchange with thewater flowing through the second branching water tubes, and not to heatexchange with the water flowing through the water inlet tube. Herein,the first compression element and the second compression element may beeither housed within the same casing or the like and controlledtogether, or disposed separately and controlled independently of eachother.

Even if the intention is to warm the water passing through the watertube system in the first heat exchanger whose refrigerant temperature islower than the second heat exchanger, for example, the temperature willsometimes be higher than the temperature of the refrigerant flowingthrough the first heat exchanger due to the water already being warmedbefore flowing into the first heat exchanger. In this case, there is arisk that it will not be possible to cool the refrigerant by heatexchange in the first heat exchanger, and that the water will bedeprived of its heat by the refrigerant.

As a countermeasure to this, in this refrigeration apparatus, the secondheat exchanger does not subject the refrigerant passing through thesecond refrigerant tube to heat exchange with the water flowing throughthe water inlet tube. Therefore, the water can be made to flow in duringa state of low temperature in which not only is the water flowing intothe second heat exchanger not yet heated by heat exchange with therefrigerant, but neither is the water flowing into the first heatexchanger.

Thereby, the refrigerant heading from the first compression elementtoward the second compression element is cooled to reliably improvecompression efficiency, heat exchanging which can raise the watertemperature can be reliably performed by both the first heat exchangerand the second heat exchanger, and the coefficient of performance of therefrigeration apparatus can be improved.

A refrigeration apparatus according to a second aspect of the presentinvention is the refrigeration apparatus according to the first aspectof the present invention, further comprising a flow rate ratioadjustment mechanism capable of adjusting the ratio between a quantityof water flowing through the first branching water tubes and a quantityof water flowing through the second branching water tubes.

According to this refrigeration apparatus, since it is possible toadjust the flow rate ratio between the quantity of water flowing throughthe first heat exchanger and the quantity of water flowing through thesecond heat exchanger, it is possible for the water to be heatedefficiently.

A refrigeration apparatus according to a third aspect of the presentinvention is the refrigeration apparatus according to the second aspectof the present invention, further comprising a heating capacitydetection unit and a water distribution quantity control unit. Theheating capacity detection unit is capable of detecting the capacity ofthe refrigerant passing through the first heat exchanger to heat thewater and the capacity of the refrigerant passing through the secondheat exchanger to heat the water. The water distribution quantitycontrol unit adjusts the ratio between the quantity of water flowingthrough the first branching water tubes and the quantity of waterflowing through the second branching water tubes by controlling the flowrate adjustment mechanism in accordance with the ratio between theheating capacities of the first heat exchanger and the second heatexchanger detected by the heating capacity detection unit. The controlby this water distribution quantity control unit may involve, forexample, adjusting the ratio between the quantity of water flowingthrough the first branching water tubes and the quantity of waterflowing through the second branching water tubes, either so as toachieve equality with the ratio between a first specific enthalpyobtained by subtracting the specific enthalpy of the refrigerantdischarged from the first compression element from the specific enthalpyof the refrigerant drawn into the second compression element, and asecond specific enthalpy obtained by subtracting the specific enthalpyof the refrigerant drawn into the second compression element from thespecific enthalpy of the refrigerant discharged from the secondcompression element, or so as to approach this ratio. The control bythis water distribution quantity control unit may otherwise involve, forexample, adjusting the ratio between the quantity of water flowingthrough the first branching water tubes and the quantity of waterflowing through the second branching water tubes, so that the watertemperature in the outlet of the first heat exchanger in the firstbranching water tubes and the water temperature in the outlet of thesecond heat exchanger in the second branching water tubes aresubstantially equal.

In this refrigeration apparatus, heat exchange for cooling therefrigerant and heating the water can be performed by both the firstheat exchanger and the second heat exchanger, and flow rate control forimproving the coefficient of performance of the refrigeration apparatuscan be performed automatically.

A refrigeration apparatus according to a fourth aspect of the presentinvention is the refrigeration apparatus according to any of the firstthrough third aspects of the present invention, wherein the secondrefrigerant tubes have a third refrigerant tube for connecting thesecond heat exchanger and the main expansion mechanism. Therefrigeration apparatus further comprises a fourth refrigerant tube forconnecting the evaporator and an intake side of the first compressionelement, a third heat exchanger for performing heat exchange between therefrigerant flowing through the third refrigerant tube and therefrigerant flowing through the fourth refrigerant tube, a third heatexchange bypass tube for connecting one end and another end of a portionof the third refrigerant tube that passes through the third heatexchanger, and a heat exchanger switching mechanism capable of switchingbetween a state in which refrigerant flows through the portion of thethird refrigerant tube that passes through the third heat exchanger, anda state in which refrigerant flows through the third heat exchangebypass tube.

In this refrigeration apparatus, through heat exchange in the third heatexchanger, the coefficient of performance can be improved by raising thedegree of supercooling of the refrigerant headed to the main expansionmechanism. Furthermore, through heat exchange in the third heatexchanger, the refrigerant drawn into the first compression element canbe subjected to an appropriate amount of superheating, liquidcompression in the first compression element can be suppressed, and thedischarge temperature can be increased to keep the resulting watertemperature high.

A refrigeration apparatus according to a fifth aspect of the presentinvention is the refrigeration apparatus according to the fourth aspectof the present invention, further comprising temperature sensory unitsand a heat exchange quantity control unit. The temperature sensory unitssense at least the air temperature surrounding the evaporator or thedischarged refrigerant temperature of at least the first compressionelement or the second compression element. The heat exchange quantitycontrol unit controls the heat exchanger switching mechanism andincreases the quantity of refrigerant flowing through the portion of thethird refrigerant tube that passes through the third heat exchanger whenthe following condition is fulfilled: the air temperature is higher thana predetermined high-temperature air temperature when the value sensedby the temperature sensory units is an air temperature, or therefrigerant temperature is lower than a predetermined low-temperaturerefrigerant temperature when the value sensed by the temperature sensoryunits is a refrigerant temperature.

In this refrigeration apparatus, the quantity of refrigerant flowingthrough the portion of the third refrigerant tube that passes throughthe third heat exchanger can be increased even in cases in which itappears that the air temperature surrounding the evaporator couldincrease or that the temperature of the refrigerant discharged from thecompression element could decrease.

It is thereby possible to increase the degree of supercooling of therefrigerant headed to the main expansion mechanism, and to improve thecoefficient of performance.

Since the refrigerant drawn into the first compression element can besubjected to the appropriate degree of superheating, it is possible toimpede liquid compression from occurring in the first compressionelement.

Furthermore, since the degree of superheating of the refrigerant drawninto the first compression element can be increased, it is possible toconform to cases in which the temperature required in the radiator ishigh.

A refrigeration apparatus according to a sixth aspect of the presentinvention is the refrigeration apparatus according to any of the firstthrough third aspects of the present invention, wherein the secondrefrigerant tubes have a third refrigerant tube connecting the secondheat exchanger and the main expansion mechanism. The refrigerationapparatus further comprises a branching expansion mechanism, a fifthrefrigerant tube, sixth refrigerant tubes, and a fourth heat exchanger.The branching expansion mechanism depressurizes refrigerant. The fifthrefrigerant tube branches off from the third refrigerant tube andextends to the branching expansion mechanism. The sixth refrigeranttubes extend from the branching expansion mechanism to the firstrefrigerant tube. The fourth heat exchanger performs heat exchangebetween refrigerant flowing through the third refrigerant tube andrefrigerant flowing through the sixth refrigerant tubes.

With this refrigeration apparatus, it is possible to improve thecoefficient of performance by raising the degree of supercooling of therefrigerant heading to the branching expansion mechanism.

When the temperature of the refrigerant being mixed in from the sixthrefrigerant tubes is lower than the temperature of the refrigerantflowing through the first refrigerant tube, it is also possible tosuppress excessive increases in the discharged refrigerant temperatureof the second compression element.

Furthermore, the quantity of refrigerant passing through the second heatexchanger can be increased.

A refrigeration apparatus according to a seventh aspect of the presentinvention is the refrigeration apparatus according to the sixth aspectof the present invention, further comprising temperature sensory unitsand a branched quantity control unit. The temperature sensory unitssense at least the air temperature surrounding the evaporator or thedischarged refrigerant temperature of at least the first compressionelement or the second compression element. The branched quantity controlunit controls the branching expansion mechanism and increases thequantity of refrigerant passing through when the following condition isfulfilled: the air temperature is lower than a predeterminedlow-temperature air temperature when the value sensed by the temperaturesensory units is an air temperature, or the refrigerant temperature ishigher than a predetermined high-temperature refrigerant temperaturewhen the value sensed by the temperature sensory units is a refrigeranttemperature. The control by the branched quantity control unit forincreasing the quantity of refrigerant passing through the branchingexpansion mechanism herein includes control for creating a flow fromconditions of a flow rate of zero (no flow), for example.

With this refrigeration apparatus, even in cases in which thetemperature of the refrigerant discharged from the first compressionelement or the second compression element will presumably increase orcases in which the air temperature surrounding the evaporator decreases,excessive increases in the discharged refrigerant temperature of thesecond compression element can be suppressed by increasing the quantityof refrigerant passing through the branching expansion mechanism, and itis possible to improve the reliability of the first compression elementor the second compression element.

A refrigeration apparatus according to an eighth aspect of the presentinvention is the refrigeration apparatus according to the sixth orseventh aspect of the present invention, further comprising a watertemperature sensory unit, a first refrigerant temperature sensory unit,and a refrigerant distribution quantity control unit. The watertemperature sensory unit senses the temperature of water flowing throughany position in the water tube system. The first refrigerant temperaturesensory unit senses the temperature of refrigerant passing through thefirst refrigerant tube. The refrigerant distribution quantity controlunit controls the branching expansion mechanism and increases thequantity of refrigerant passing through when the difference between thetemperature sensed by the water temperature sensory unit and thetemperature sensed by the first refrigerant temperature sensory unit isless than a predetermined value.

With this refrigeration apparatus, even when the water's effect ofcooling the refrigerant flowing through the first refrigerant tube isinsufficient, it is possible to improve the coefficient of performanceof the refrigeration cycle by causing the sixth refrigerant tubes toconverge and thereby lowering the temperature of the refrigerant passingthrough the first refrigerant tube.

A refrigeration apparatus according to a ninth aspect of the presentinvention is the refrigeration apparatus according to any of the firstthrough third aspects of the present invention, wherein the secondrefrigerant tubes have a third refrigerant tube connecting the secondheat exchanger and the main expansion mechanism. The refrigerationapparatus further comprises a branching expansion mechanism, a fourthrefrigerant tube, a third heat exchanger, a fifth refrigerant tube,sixth refrigerant tubes, and a fourth heat exchanger. The branchingexpansion mechanism depressurizes refrigerant. The fourth refrigeranttubes connect the evaporator and an intake side of the first compressionelement. The third heat exchanger performs heat exchange betweenrefrigerant flowing through the third refrigerant tube and refrigerantflowing through the fourth refrigerant tubes. The fifth refrigerant tubebranches off from the third refrigerant tube and extends to thebranching expansion mechanism. The sixth refrigerant tubes connect thebranching expansion mechanism and the first refrigerant tube. The fourthheat exchanger performs heat exchange between refrigerant flowingthrough the third refrigerant tube and refrigerant flowing through thesixth refrigerant tubes.

With this refrigeration apparatus, it is possible to raise the degree ofsupercooling of the refrigerant heading to the branching expansionmechanism and improve the coefficient of performance, and to apply theappropriate amount of heating to the refrigerant drawn into the firstcompression element and prevent liquid compression in the firstcompression element and/or cool the refrigerant flowing through thethird refrigerant tube.

A refrigeration apparatus according to a tenth aspect of the presentinvention is the refrigeration apparatus according to the ninth aspectof the present invention, further comprising temperature sensory unitsand a branched heat quantity control unit. The temperature sensory unitssense at least the air temperature surrounding the evaporator or thedischarged refrigerant temperature of at least the first compressionelement or the second compression element. The branched heat quantitycontrol unit controls the branching expansion mechanism and increasesthe quantity of refrigerant passing through when the following conditionis fulfilled: the air temperature is lower than a predeterminedlow-temperature air temperature when the value sensed by the temperaturesensory units is an air temperature, or the refrigerant temperature ishigher than a predetermined high-temperature refrigerant temperaturewhen the value sensed by the temperature sensory units is a refrigeranttemperature. The control by the branched quantity control unit forincreasing the quantity of refrigerant passing through the branchingexpansion mechanism herein includes control for creating a flow fromconditions of a flow rate of zero (no flow), for example.

With this refrigeration apparatus, even in cases in which thetemperature of the refrigerant discharged from the compression elementwill presumably increase or cases in which the air temperaturesurrounding the evaporator decreases, excessive increases in thedischarged refrigerant temperature of the second compression element canbe suppressed by increasing the quantity of refrigerant passing throughthe branching expansion mechanism, and it is possible to improve thereliability of the second compression element.

A refrigeration apparatus according to an eleventh aspect of the presentinvention is the refrigeration apparatus according to the ninth or tenthaspect of the present invention, further comprising a first heatexchange bypass tube and a bypass switching mechanism. The first heatexchange bypass tube connects one end and another end of the portion ofthe first refrigerant tube that passes through the first heat exchanger.The bypass switching mechanism is capable of switching between a statein which refrigerant flows through the portion of the first refrigeranttube that passes through the first heat exchanger, and a state in whichrefrigerant flows through the first heat exchange bypass tube.

With this refrigeration apparatus, in the first heat exchanger, theswitching of the bypass switching mechanism makes it possible to switchbetween a state of allowing and a state of not allowing the passage ofrefrigerant in the heat exchange bypass tube, and also to adjust theusage condition of the first heat exchanger.

A refrigeration apparatus according to a twelfth aspect of the presentinvention is the refrigeration apparatus according to the eleventhaspect of the present invention, further comprising temperature sensoryunits and a bypass control unit. The temperature sensory units sense atleast the air temperature surrounding the evaporator or the dischargedrefrigerant temperature of at least the first compression element or thesecond compression element. The bypass control unit controls the bypassswitching mechanism and increases the quantity of refrigerant flowingthrough the portion of the first refrigerant tube that passes throughthe first heat exchanger when the following condition is fulfilled: theair temperature is higher than a predetermined high-temperature airtemperature when the value sensed by the temperature sensory units is anair temperature, or the refrigerant temperature is lower than apredetermined low-temperature refrigerant temperature when the valuesensed by the temperature sensory units is a refrigerant temperature.The control by the bypass control unit for increasing the quantity ofrefrigerant passing through the portion of the first refrigerant tubeherein includes control for creating a flow from conditions of a flowrate of zero (no flow), for example.

With this refrigeration apparatus, even in cases in which thetemperature of the refrigerant discharged from the compression elementwill presumably be low or cases in which the air temperature surroundingthe evaporator has increased, the degree of superheating of therefrigerant drawn into the second compression element can be raised byreducing the quantity of refrigerant flowing through the portion of thefirst refrigerant tube that passes through the first heat exchanger, andit is possible to comply with a high temperature requirement in theradiator.

A refrigeration apparatus according to a thirteenth aspect of thepresent invention is the refrigeration apparatus according to any of theninth through twelfth aspects of the present invention, furthercomprising a water temperature sensory unit, a first refrigeranttemperature sensory unit, and a water-correspondent refrigerant quantitycontrol unit. The water temperature sensory unit senses the temperatureof water flowing through any position in the water tube system. Thefirst refrigerant temperature sensory unit senses the temperature ofrefrigerant passing through the first refrigerant tube. Thewater-correspondent refrigerant quantity control unit controls thebranching expansion mechanism and increases the quantity of refrigerantpassing through when the difference between the temperature sensed bythe water temperature sensory unit and the temperature sensed by thefirst refrigerant temperature sensory unit is less than a predeterminedvalue.

With this refrigeration apparatus, even when the water's effect ofcooling the refrigerant flowing through the first refrigerant tube isinsufficient, it is possible to improve the coefficient of performanceof the refrigeration cycle by causing the refrigerant passing throughthe sixth refrigerant tubes to mix, thereby lowering the temperature ofthe refrigerant flowing through the first refrigerant tube.

A refrigeration apparatus according to a fourteenth aspect of thepresent invention is the refrigeration apparatus according to any of thefirst through thirteenth aspects of the present invention, furthercomprising a first drive unit for driving the first compression element,and a second drive unit for driving the second compression elementindependently of the first compression element.

With this refrigeration apparatus, since the capacity of the firstcompression element and the capacity of the second compression elementcan be adjusted so as to be different, during control for mixing thewater flowing through the first branching water tube and the waterflowing through the second branching water tube and attempting to bringthe temperature of the water flowing through the output water tube to atarget temperature, the coefficient of performance can be madesatisfactory and the effect of minimizing the compression work can befurther improved by separately adjusting the capacity of the firstcompression element and the capacity of the second compression element.

A refrigeration apparatus according to a fifteenth aspect of the presentinvention is the refrigeration apparatus according to any of the firstthrough thirteenth aspects of the present invention, wherein the firstcompression element and the second compression element have a sharedrotating shaft for performing compression work by rotatably driving eachof the compression elements.

With this refrigeration apparatus, vibrations and/or fluctuations intorque load can be suppressed by driving the compression elements whilecausing their centrifugal forces to cancel each other out.

A refrigeration apparatus according to a sixteenth aspect of the presentinvention is the refrigeration apparatus according to any of the firstthrough fifteenth aspects of the present invention, wherein the activerefrigerant is carbon dioxide.

In this refrigeration apparatus, with carbon dioxide in a supercriticalstate near a critical point, the density of the refrigerant can bechanged dramatically merely by slightly changing the refrigerantpressure. Therefore, the efficiency of the refrigeration apparatus canbe improved by a small amount of compression work.

ADVANTAGEOUS EFFECTS OF INVENTION

As stated in the above descriptions, the following effects are achievedaccording to the present invention.

In the first aspect, the refrigerant heading from the first compressionelement toward the second compression element is cooled to reliablyimprove compression efficiency, heat exchanging which can raise thewater temperature can be reliably performed by both the first heatexchanger and the second heat exchanger, and the coefficient ofperformance of the refrigeration apparatus can be improved.

In the second aspect, it is possible for the water to be heatedefficiently.

In the third aspect, flow rate control for improving the coefficient ofperformance of the refrigeration apparatus can be performedautomatically.

In the fourth aspect, it is possible to improve the coefficient ofperformance, to suppress liquid compression in the first compressionelement, and to increase the discharge temperature to keep the resultingwater temperature high.

In the fifth aspect, it is possible to increase the degree ofsupercooling of the refrigerant headed to the main expansion mechanism,and to improve the coefficient of performance.

In the sixth aspect, it is possible to improve the coefficient ofperformance by raising the degree of supercooling of the refrigerantheading to the branching expansion mechanism.

In the seventh aspect, it is possible to improve the reliability of thefirst compression element or the second compression element.

In the eighth aspect, even when the water's effect of cooling therefrigerant flowing through the first refrigerant tube is insufficient,it is possible to improve the coefficient of performance of therefrigeration cycle.

In the ninth aspect, it is possible to improve the coefficient ofperformance, and to prevent liquid compression in the first compressionelement and/or cool the refrigerant flowing through the thirdrefrigerant tube.

In the tenth aspect, even in cases in which the temperature of therefrigerant discharged from the compression element will presumablyincrease or cases in which the air temperature surrounding theevaporator decreases, it is possible to improve the reliability of thesecond compression element.

In the eleventh aspect, it is possible to switch between a state ofallowing and a state of not allowing the passage of refrigerant in theheat exchange bypass tube, and also to adjust the usage condition.

In the twelfth aspect, even in cases in which the temperature of therefrigerant discharged from the compression element will presumably below or cases in which the air temperature surrounding the evaporator hasincreased, it is possible to comply with a high temperature requirementin the radiator.

In the thirteenth aspect, even when the water's effect of cooling therefrigerant flowing through the first refrigerant tube is insufficient,it is possible to improve the coefficient of performance of therefrigeration cycle.

In the fourteenth aspect, the coefficient of performance can be madesatisfactory and the effect of minimizing the compression work can befurther improved.

In the fifteenth aspect, the occurrence of vibrations and/orfluctuations in torque load can be suppressed by driving the compressionelements while causing their centrifugal forces to cancel each otherout.

In the sixteenth aspect, the efficiency of the refrigeration apparatuscan be improved by a small amount of compression work.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic structural diagram of an air-conditioningapparatus as an embodiment of the refrigeration apparatus according tothe first embodiment of the present invention.

FIG. 2 is a pressure-enthalpy graph representing the refrigeration cycleof the air-conditioning apparatus according to the first embodiment.

FIG. 3 is a temperature-entropy graph representing the refrigerationcycle of the air-conditioning apparatus according to the firstembodiment.

FIG. 4 is a schematic structural diagram of an air-conditioningapparatus according to Modification 1 of the first embodiment.

FIG. 5 is a schematic structural diagram of an air-conditioningapparatus according to Modification 2 of the first embodiment.

FIG. 6 is a schematic structural diagram of an air-conditioningapparatus according to Modification 3 of the first embodiment.

FIG. 7 is a schematic structural diagram of an air-conditioningapparatus according to Modification 7 of the first embodiment.

FIG. 8 is a schematic structural diagram of an air-conditioningapparatus as an embodiment of the refrigeration apparatus according tothe second embodiment of the present invention.

FIG. 9 is a pressure-enthalpy graph representing the refrigeration cycleof the air-conditioning apparatus according to the second embodiment.

FIG. 10 is a temperature-entropy graph representing the refrigerationcycle of the air-conditioning apparatus according to the secondembodiment.

FIG. 11 is a schematic structural diagram of an air-conditioningapparatus according to Modification 1 of the second embodiment.

FIG. 12 is a schematic structural diagram of an air-conditioningapparatus according to Modification 5 of the second embodiment.

FIG. 13 is a schematic structural diagram of an air-conditioningapparatus as an embodiment of a refrigeration apparatus according to thethird embodiment of the present invention.

FIG. 14 is a pressure-enthalpy graph representing the refrigerationcycle of an air-conditioning apparatus according to the thirdembodiment.

FIG. 15 is a temperature-entropy graph representing the refrigerationcycle of an air-conditioning apparatus according to the thirdembodiment.

FIG. 16 is a schematic structural diagram of an air-conditioningapparatus according to Modification 1 of the third embodiment.

FIG. 17 is a schematic structural diagram of an air-conditioningapparatus according to Modification 4 of the third embodiment.

FIG. 18 is a schematic structural diagram of an air-conditioningapparatus as an embodiment of a refrigeration apparatus according to thefourth embodiment of the present invention.

FIG. 19 is a pressure-enthalpy graph representing the refrigerationcycle of an air-conditioning apparatus according to the fourthembodiment.

FIG. 20 is a temperature-entropy graph representing the refrigerationcycle of an air-conditioning apparatus according to the fourthembodiment.

FIG. 21 is a schematic structural diagram of an air-conditioningapparatus according to Modification 1 of the fourth embodiment.

FIG. 22 is a schematic structural diagram of an air-conditioningapparatus according to Modification 2 of the fourth embodiment.

FIG. 23 is a schematic structural diagram of an air-conditioningapparatus according to Modification 3 of the fourth embodiment.

FIG. 24 is a schematic structural diagram of an air-conditioningapparatus according to Modification 4 of the fourth embodiment.

FIG. 25 is a schematic structural diagram of an air-conditioningapparatus according to Modification 8 of the fourth embodiment.

DESCRIPTION OF EMBODIMENTS <1> First Embodiment <1-1> Configuration ofAir-Conditioning Apparatus

FIG. 1 is a schematic structural diagram of a water heater 1 as anembodiment of the refrigeration apparatus according to the presentinvention. The water heater 1 is an apparatus for producing heated waterby using refrigerant that operates in a supercritical range (carbondioxide in this case) to perform a two-stage compression-typerefrigeration cycle.

The water heater 1 has a water circuit 910 and a refrigerant circuit 10.

(Water Circuit)

The water circuit 910 has a water inlet tube 901 for leading watersupplied from the exterior to a water branching point W, heat sourcewater tubes 902, 903 and intermediate water tubes 904, 905 extendingfrom the branching point W, and a water outlet tube 906 leading out tothe exterior from a convergent point Z where the heat source water tubes902, 903 and the intermediate water tubes 904, 905 converge, as shown inFIG. 1.

A pump 921 capable of adjusting the quantity of water passing through isprovided to the water inlet tube 901. This pump 921 is provided with amotor 921 m, the rotational speed is adjusted by a control unit 99, andthe flow rate of the pump is adjusted. The water circuit 910 is alsoprovided with a water temperature sensor 910T for detecting thetemperature of the water passing through the water inlet tube 901.Through the water temperature sensor 910T, the control unit 99 canperceive the temperature of the supplied water, the control unit 99perceives the difference from the output water temperature requested bythe user, and a refrigeration cycle of the refrigerant circuit 10 isadjusted.

The heat source water tube 902 extends from the branching point W to aheat source-side heat exchanger 4 of the refrigerant circuit 10,described hereinafter. The heat source water tube 903 extends so as tolead water flowing out of the heat source-side heat exchanger 4 to theconvergent point Z. Thus, the water flowing through the heat sourcewater tubes 902, 903 is warmed in the heat source-side heat exchanger 4by heat exchange with the refrigerant flowing through the refrigerantcircuit 10, and heated water is produced. A tube configuration iscreated so that the refrigerant and water flow opposite each other in anintercooler 7, and the heat exchange efficiency is improved.

The intermediate water tube 904 extends from the branching point W tothe intercooler 7 of the refrigerant circuit 10, described hereinafter.The intermediate water tube 905 extends so as to lead the water flowingout of the intercooler 7 to the convergent point Z. Thus, the waterflowing through the intermediate water tubes 904, 905 is warmed in theintercooler 7 by heat exchange with the refrigerant flowing through therefrigerant circuit 10, and heated water is produced. A tubeconfiguration is created so that the refrigerant and water flow oppositeeach other in the heat source-side heat exchanger 4, and the heatexchange efficiency is improved. Since the refrigerant temperature inthe intermediate water tubes 904, 905, which is the target of heatexchange, is lower than the refrigerant temperature in the heatsource-side heat exchanger 4, the intermediate water tubes 904, 905 aredesigned to be smaller in diameter than the heat source water tubes 902,903 in order to ensure that the water heating will take place primarilyin the heat source-side heat exchanger 4.

Hot water which has been warmed in the heat source water tubes 902, 903and in the intermediate water tubes 904, 905 and mixed at the convergentpoint Z is then supplied to the user through the water outlet tube 906.

(Refrigerant Circuit)

The refrigerant circuit 10 has primarily a low-stage compression element2 c, a high-stage compression element 2 d, the heat source-side heatexchanger 4, an expansion mechanism 5, a usage-side heat exchanger 6, anintermediate refrigerant tube 22, an intercooler 7, connection tubes 71,72, 76 or the like connecting these components, and a usage-sidetemperature sensor 6T.

In the present embodiment, the low-stage compression element 2 c and thehigh-stage compression element 2 d compress the refrigerant sequentiallyin two stages.

The low-stage compression element 2 c has a hermetically sealedstructure in which a compressor drive motor 21 b and a drive shaft 21 care housed within a casing 21 a. The compressor drive motor 21 b islinked to the drive shaft 21 c. This drive shaft 21 c is linked to thecompression element 2 c. The compression element 2 c is a rotary-type,scroll-type, or another type of positive displacement compressionelement in the present embodiment. The low-stage compression element 2 cdraws refrigerant in from an intake tube 2 a, compresses the drawn-inrefrigerant, and discharges the refrigerant toward the intermediaterefrigerant tube 22. The intermediate refrigerant tube 22 connects thedischarge side of the low-stage compression element 2 c and the intakeside of the high-stage compression element 2 d via the intercooler 7. Adischarge tube 2 b is a refrigerant tube for feeding the refrigerantdischarged from the low-stage compression element 2 c to the high-stagecompression element 2 d via the intercooler 7, and the discharge tube 2b is provided with a non-return mechanism 42 c and a mechanism forseparating the refrigerant from refrigeration oil which accompanies therefrigerant discharged from the low-stage compression element 2 c andreturning the refrigeration oil to the intake side of the low-stagecompression element 2 c. The mechanism for returning the refrigerationoil has primarily an oil separator 41 a for separating the refrigerantfrom the refrigeration oil accompanying the refrigerant discharged fromthe low-stage compression element 2 c, and an oil return tube 41 b whichis connected to the oil separator 41 a and which returns therefrigeration oil separated from the refrigerant to the intake tube 2 aof the low-stage compression element 2 c. The oil return tube 41 b isprovided with a depressurization mechanism 41 c for depressurizing therefrigeration oil flowing through the oil return tube 41 b. A capillarytube is used as the depressurization mechanism 41 c in the presentembodiment. The non-return mechanism 42 c is a mechanism for allowingthe flow of refrigerant from the discharge side of the low-stagecompression element 2 c to the intercooler 7 and blocking the flow ofrefrigerant from the intercooler 7 to the discharge side of thelow-stage compression element 2 c, and a non-return valve is used in thepresent embodiment.

The high-stage compression element 2 d is similar to the low-stagecompression element 2 c, and has a hermetically sealed structure inwhich a compressor drive motor 21 e and a drive shaft 21 f are housedwithin a casing 21 d. The compressor drive motor 21 e is linked to thedrive shaft 21 f. This drive shaft 21 f is linked to the compressionelement 2 d. The compression element 2 d is a rotary-type, scroll-type,or another type of positive displacement compression element in thepresent embodiment. The high-stage compression element 2 d drawsrefrigerant in from the intermediate refrigerant tube 22, compresses thedrawn-in refrigerant, and discharges the refrigerant toward a dischargetube 2 e. The discharge tube 2 e connects the discharge side of thehigh-stage compression element 2 d and the heat source-side heatexchanger 4. The discharge tube 2 e is provided with a non-returnmechanism 42 d and a mechanism for separating the refrigerant fromrefrigeration oil which accompanies the refrigerant discharged from thehigh-stage compression element 2 d and returning the refrigeration oilto the intake side of the high-stage compression element 2 d. Themechanism for returning the refrigeration oil has primarily an oilseparator 41 d for separating the refrigerant from the refrigeration oilaccompanying the refrigerant discharged from the high-stage compressionelement 2 d, and an oil return tube 41 e which is connected to the oilseparator 41 d and which returns the refrigeration oil separated fromthe refrigerant to the intermediate refrigerant tube 22, which is on theintake side of the high-stage compression element 2 d. The oil returntube 41 e is provided with a depressurization mechanism 41 f fordepressurizing the refrigeration oil flowing through the oil return tube41 e. A capillary tube is used as the depressurization mechanism 41 f inthe present embodiment. The non-return mechanism 42 d is a mechanism forallowing the flow of refrigerant from the discharge side of thehigh-stage compression element 2 d to the heat source-side heatexchanger 4 and blocking the flow of refrigerant from the heatsource-side heat exchanger 4 to the discharge side of the high-stagecompression element 2 d, and a non-return valve is used in the presentembodiment.

That is, the two compression elements 2 c, 2 d are connected to eachother in series and are linked to their respective individual driveshafts 21 c, 21 f, and the two compression elements 2 c, 2 d havetwo-stage compression structures in which they are rotatably drivenindividually by respective compressor drive motors 21 b, 21 e.

The intercooler 7 warms the water flowing through the intermediate watertubes 904, 905 by the heat of the refrigerant flowing through theintermediate refrigerant tube 22, and cools the refrigerant flowingthrough the intermediate refrigerant tube 22 by the water flowingthrough the intermediate water tubes 904, 905. The degree ofsuperheating of the refrigerant drawn into the high-stage compressionelement 2 d can thereby be reduced, and the temperature of therefrigerant discharged from the high-stage compression element 2 d isprevented from increasing excessively. The refrigeration capacity canalso be improved because the density of the refrigerant drawn into thehigh-stage compression element 2 d is increased by lowering thetemperature of the refrigerant flowing through the intermediaterefrigerant tube 22 in this manner.

The heat source-side heat exchanger 4 is a heat exchanger which has airas a heat source and functions as a radiator of refrigerant. The heatsource-side heat exchanger 4 is connected at one end to the dischargeside of the high-stage compression element 2 d via the connection tube71 and the non-return mechanism 42, and is connected at the other end tothe expansion mechanism 5 via the connection tube 72. In this heatsource-side heat exchanger 4, the water flowing through the heat sourcewater tubes 902, 903 is heated by the refrigerant heading from theconnection tube 71 to the connection tube 72, and the refrigerantheading from the connection tube 71 to the connection tube 72 is cooledby the water flowing through the heat source water tubes 902, 903.

The expansion mechanism 5 is connected at one end to the connection tube72, and is connected at the other end to the usage-side heat exchanger 6via the connection tube 76. The expansion mechanism 5 is a mechanism fordepressurizing the refrigerant, and an electric expansion valve is usedin the present embodiment. In the present embodiment, the expansionmechanism 5 depressurizes the high-pressure refrigerant cooled in theheat source-side heat exchanger 4 nearly to the saturation pressure ofthe refrigerant before feeding the refrigerant to the usage-side heatexchanger 6.

The usage-side heat exchanger 6 is a heat exchanger which functions asan evaporator of refrigerant. The usage-side heat exchanger 6 isconnected at one end to the expansion mechanism 5 via the connectiontube 76, and is connected at the other end to the intake side of thelow-stage compression element 2 c via the intake tube 2 a. Though notshown in the diagram, the usage-side heat exchanger 6 is supplied withwater and/or air as a heating source for performing heat exchange withthe refrigerant flowing through the usage-side heat exchanger 6.

The usage-side temperature sensor 6T detects the temperature of thewater and/or air supplied as a heating source in order to perform heatexchange with the refrigerant flowing through the usage-side heatexchanger 6 described above.

As described above, the water heater 1 is provided with a control unit99 which perceives the temperature sensed by the usage-side temperaturesensor 6T, and which controls the actions of the low-stage compressionelement 2 c, the high-stage compression element 2 d, the expansionmechanism 5, the pump 921, and other components constituting the waterheater 1.

<1-2> Action of Air-Conditioning Apparatus

Next, the action of the water heater 1 of the present embodiment will bedescribed using FIGS. 1, 2 and 3.

Herein, FIG. 2 is a pressure-enthalpy graph representing therefrigeration cycle, and FIG. 3 is a temperature-entropy graphrepresenting the refrigeration cycle.

The states of the refrigerant at the points indicated by A, B, C, D, K,and M in the refrigerant circuit 10 of FIG. 1 correspond to the samepoints in the pressure-enthalpy graph shown in FIG. 2 and thetemperature-entropy graph shown in FIG. 3.

In this refrigeration cycle, the refrigerant flowing through theintermediate refrigerant tube 22 is cooled by the refrigerant flowingthrough the intermediate water tubes 904, 905 of the water circuit 910when passing through the intercooler 7 (refer to point B point C inFIGS. 2 and 3).

(Target Capacity Output Control)

In this type of refrigeration cycle, the control unit 99 performs targetcapacity output control as follows.

First, the control unit 99 receives an input value of output watertemperature and an input value of a required output water quantity fromthe user via a controller or the like (not shown). The control unit 99controls the flow rate of water by controlling the rotational speed ofthe motor 921 m of the pump 921 on the basis of the input value of therequired output water quantity.

The control unit 99 then perceives the water temperature detected by thewater temperature sensor 910T and the flow rate controlled by the motor921 m of the pump 921, and calculates emitted heat quantity required forthe refrigerant supplied to the heat source-side heat exchanger 4. Basedon this required emitted heat quantity, the control unit 99 thencalculates the target discharge pressure for the pressure of therefrigerant discharged from the high-stage compression element 2 d.

A case of a target discharge pressure being the target value in thetarget capacity output control is herein described as an example, butinstead of the target discharge pressure, target values of thedischarged refrigerant pressure and the discharged refrigeranttemperature may be established so that the value obtained by multiplyingthe discharged refrigerant temperature by the discharged refrigerantpressure is within a predetermined range. This is because in cases inwhich the load has changed, the density of the discharged refrigerantdecreases when the degree of superheating of the drawn-in refrigerant ishigh; therefore, even if it is possible to maintain the temperature ofthe refrigerant discharged from the high-stage compression element 2 d,it becomes impossible to guarantee the emitted heat quantity required inthe heat source-side heat exchanger 4.

Next, based on the temperature sensed by the usage-side temperaturesensor 6T, the control unit 99 establishes a target evaporationtemperature and a target evaporation pressure (a pressure equal to orless than the critical pressure). This setting of the target evaporationpressure is performed every time the temperature sensed by theusage-side temperature sensor 6T changes.

Based on this target evaporation temperature value, the control unit 99performs superheat degree control so that the degree of superheating ofthe refrigerant drawn in by the low-stage compression element 2 c is atarget value x (degree of superheating target value) of 5° C. or less.

The control unit 99 then controls the low-stage compression element 2 cso as to raise the refrigerant pressure and refrigerant temperaturewhile causing isentropic change for maintaining the entropy value at thedegree of superheating established in this manner, and refrigerant isdischarged to the intermediate refrigerant tube 22. In the intercooler 7provided to the intermediate refrigerant tube 22, heat exchange isperformed while the water and refrigerant flow against each other, therefrigerant is cooled by the water flowing through the intermediatewater tubes 904, 905, and the water flowing through the intermediatewater tubes 904, 905 is heated. Thus, the refrigerant flowing throughthe intermediate refrigerant tube 22 is cooled in the intercooler 7 anddrawn into the high-stage compression element 2 d. In the high-stagecompression element 2 d, refrigerant is discharged at a pressureexceeding the critical pressure due to the operating capacity beingcontrolled by controlling the rotational speed. Having been increased intemperature by being further compressed by the high-stage compressionelement 2 d in this manner, the refrigerant is fed to the heatsource-side heat exchanger 4. In the heat source-side heat exchanger 4,heat exchange is performed while the high-temperature, high-pressurerefrigerant in a supercritical state and the water flow against eachother, and water having the target output water temperature is obtained.

In this heat radiation process in the heat source-side heat exchanger 4,since the refrigerant is in a supercritical state, the refrigeranttemperature continuously decreases while the pressure is being changedsuch that the refrigerant is maintained at the target discharge pressureas an isobaric change. The refrigerant flowing through the heatsource-side heat exchanger 4 has a temperature equal to or greater thanthe temperature of the water supplied as a heating target through theheat source water tubes 902, 903, and the refrigerant is cooled to avalue y near the water supplied as a heating target. The value of ychanges due to the supplied quantity being controlled by the motor 921 mof the pump 921.

The refrigerant cooled in the heat source-side heat exchanger 4 in thismanner is depressurized to the target evaporation pressure (a pressureequal to or less than the critical pressure) by the expansion mechanism5, and the refrigerant flows into the usage-side heat exchanger 6.

The refrigerant flowing through the usage-side heat exchanger 6 absorbsheat from the water and/or air supplied as a heating source, whereby thedryness of the refrigerant is progressively improved whileisobaric-isothermal change is conducted such that the target evaporationtemperature and the target evaporation pressure are maintained. Thecontrol unit 99 then controls the quantity supplied by a heating sourcesupply device (not shown) (a pump in the case of water, and a fan or thelike in the case of air) so that the degree of superheating reaches thedegree of superheating target value.

When performing control in this manner, the control unit 99 calculatesthe value of x and the value of y so that the coefficient of performance(COP) in the refrigeration cycle will be as high as possible, andperforms the target capacity output control described above. Whencalculating the value of x and the value of y which yield the bestcoefficient of performance, the control unit 99 performs the calculationon the basis of the properties (a Mollier diagram or the like) of carbondioxide as the active refrigerant. A condition may be established atwhich the coefficient of performance can be satisfactorily maintained toa certain extent, and if this condition is met, the value of x and thevalue of y may be determined such that the compression work will be asmaller value. Another option is to use a precondition that thecompression work is suppressed to a predetermined value or lower, and todetermine the value of x and the value of y which yield the bestcoefficient of performance while this precondition is being met.

The relationship between heat radiation quantity control guaranteed inthe heat source-side heat exchanger 4 and/or the intermediaterefrigerant tube 22 of the refrigerant circuit 10 and flow rate controlof the pump 921, which are performed by the control unit 99, includes alarge adjustment of the flow rate of the pump 921 of the water circuit910 or another action when the refrigerant circuit 10 is controlled sothat the heat radiation quantity in the heat source-side heat exchanger4 and/or the intermediate refrigerant tube 22 increases, for example.This relationship between heat radiation quantity control and flow ratecontrol also includes control for minimizing the flow rate of the pump921 of the water circuit 910 when, conversely, a large heat radiationquantity cannot be achieved in the heat source-side heat exchanger 4and/or the intermediate refrigerant tube 22. The control unit 99 givesless priority to controlling the output water quantity than to theoutput water temperature while achieving the output water temperaturerequested by the user.

(Characteristics of First Embodiment)

The water that has branched off in the branching point W and that flowsthrough the intermediate water tubes 904, 905 is water that has not beenheated in the heat source-side heat exchanger 4, and this water has thesame temperature as the temperature of the refrigerant flowing throughthe water inlet tube 901. During control performed by the control unit99, in the refrigerant circuit 10 which uses carbon dioxide as theactive refrigerant, degree of superheating control is performed so thatthe degree of superheating reaches a target value x of 5° C. or less,and the compression ratio in the low-stage compression element 2 c andthe compression ratio in the high-stage compression element 2 d areadjusted so as to be equal. Therefore, according to the properties ofcarbon dioxide in a Mollier diagram, the temperature of the refrigerantflowing through the heat source-side heat exchanger 4 is inevitablyhigher than the temperature of the refrigerant flowing through theintercooler 7. Thus, in a refrigeration cycle in which the temperatureof the refrigerant flowing through the intercooler 7 and the temperatureof the refrigerant flowing through the heat source-side heat exchanger 4are different and the temperature of the refrigerant flowing through theintercooler 7 is the lower of the two, in order to achieve therefrigeration cycle effect by way of the intercooler 7, i.e., effect ofcooling the refrigerant drawn into the high-stage compression element 2d, the refrigeration cycle is limited to cases in which the temperatureof the water flowing through the intermediate water tubes 904, 905 islower than the temperature of the refrigerant flowing through theintermediate refrigerant tube 22. Therefore, if the water flowing intothe intercooler 7 has already been heated to a certain extent in theheat source-side heat exchanger 4 before flowing into the intercooler 7,not only is this effect of cooling the refrigerant drawn into thehigh-stage compression element 2 d lessened, but the reverse effect willsometimes occur when the temperature of the water flowing through theintermediate water tubes 904, 905 is higher than the temperature of therefrigerant flowing through the intermediate refrigerant tube 22. As acountermeasure to this, in the water heater 1 of the present embodiment,the water flowing into the intercooler 7 can be made to flow into theintercooler 7 via the intermediate water tube 904 without taking heatfrom the exterior after the water has branched off at the branchingpoint W. Therefore, the effect of cooling the refrigerant drawn into thehigh-stage compression element 2 d can be achieved merely due to thetemperature of the water flowing through the water inlet tube 901 atleast being lower than the temperature of the refrigerant flowingthrough the intermediate refrigerant tube 22, and it is possible toprevent this effect from lessening or to prevent the reverse effect fromoccurring due to the water flowing through the intermediate water tubes904, 905 being heated before flowing into the intercooler 7.

<1-3> Modification 1

As one example of control by the control unit 99 in the embodimentdescribed above, the following type of control can be performed, forexample.

In this example, the design pressure resistance of the low-stagecompression element 2 c and the high-stage compression element 2 d is 12MPa, and the discharged refrigerant pressure must be maintained at orbelow this design pressure resistance.

In cases in which the control unit 99 establishes the dischargedrefrigerant temperature on the basis of the input value of the outputwater temperature from the user, the target discharge pressure andtarget discharge temperature are established so as to achieve arefrigerant pressure and refrigerant temperature which are equal to orless than the design pressure described above and which can guarantee aheat radiation quantity of the refrigerant supplied to the heatsource-side heat exchanger 4. The target state of the dischargedrefrigerant thereby converges at one point in a Mollier diagram of therefrigeration cycle which uses carbon dioxide as the active refrigerant.

On the other hand, the target evaporation pressure of the refrigerationcycle is established according to the temperature sensed by theusage-side temperature sensor 6T.

In cases in which a control is performed so that the compression ratioin the low-stage compression element 2 c and the compression ratio inthe high-stage compression element 2 d are equal, the targetintermediate pressure flowing through the intermediate refrigerant tube22 is established according to the above-described relationship betweenthe target discharge pressure and the evaporation pressure.

When the degree of superheating of the refrigerant drawn into thelow-stage compression element 2 c is set at 5° C., the dischargedrefrigerant temperature and pressure are established by causingisentropic change in the low-stage compression element 2 c. Furthermore,if isentropic change takes place in the high-stage compression element 2d so that the target discharge pressure and target discharge temperaturecan be achieved, the temperature of the refrigerant drawn into thehigh-stage compression element 2 d is established.

The quantity of cold energy needed to cool the refrigerant dischargedfrom the low-stage compression element 2 c until it is drawn into thehigh-stage compression element 2 d is thereby established, and thecontrol unit 99 may control the flow rate of the pump 921 on the basisof the value detected by the water temperature sensor 910T so that thiscold energy quantity can be supplied.

The degree of superheating is not limited to 5° C., and can be selectedwithin a range of 0 to 5° C. The temperature of the refrigerant flowinginto the expansion mechanism 5 is also adjustable, and the value of xand the value of y may be established so as to yield the bestcoefficient of performance of the refrigeration cycle while these valuesare being adjusted.

<1-4> Modification 2

As shown in FIG. 4, for example, a refrigerant circuit 10A may be usedwhich has a discharged refrigerant temperature sensor 2T for sensing thetemperature of the refrigerant discharged from the high-stagecompression element 2 d.

If the temperature detected by this discharged refrigerant temperaturesensor 2T is too high, it will not be possible to maintain thereliability of the high-stage compression element 2 d, and the controlunit 99 may therefore perform control for reducing the flow rate of thepump 921 while reducing the discharged refrigerant temperature.

It is thereby possible to achieve the output water temperature requestedby the user while guaranteeing the reliability of the high-stagecompression element 2 d.

<1-5> Modification 3

In the embodiment described above, an example was described in which thediameters of the intermediate water tubes 904, 905 are designed to besmaller than the diameters of the heat source water tubes 902, 903.

However, the present invention is not limited to this example, andanother option is to use a refrigerant circuit 10B provided with a flowrate ratio adjustment mechanism 911 capable of adjusting the ratiobetween the quantity of water flowing through the intermediate watertubes 904, 905 and the quantity of water flowing through the heat sourcewater tubes 902, 903, as shown in FIG. 5, for example.

For example, the flow rate ratio adjustment mechanism 911 can beprovided at an intermediate point in the intermediate water tubes 904,905. The flow rate ratio can thereby be adjusted even if the diametersof the intermediate water tubes 904, 905 and the diameters of the heatsource water tubes 902, 903 are the same.

When the flow rate ratio between the quantity of water flowing throughthe heat source water tubes 902, 903 and the quantity of water flowingthrough the intermediate water tubes 904, 905 is adjusted, the controlunit 99 may control the flow rate ratio so as to achieve equalitybetween the ratio of the heating amount in the heat source-side heatexchanger 4 and the heating amount in the intercooler 7 as obtained fromthe Mollier diagram, and the ratio of the quantity of water flowingthrough the heat source water tubes 902, 903 and the quantity of waterflowing through the intermediate water tubes 904, 905, for example. Theheating amount in the intercooler 7 in this case can be perceivedaccording to an intermediate specific enthalpy (point B→point C in theMollier diagram) obtained by subtracting the specific enthalpy of therefrigerant discharged from the low-stage compression element 2 c fromthe specific enthalpy of the refrigerant drawn into the high-stagecompression element 2 d. The heating amount in the heat source-side heatexchanger 4 can be perceived according to a heat source specificenthalpy (point D→point K in the Mollier diagram) obtained bysubtracting the specific enthalpy of the refrigerant discharged from thehigh-stage compression element 2 d from the specific enthalpy of therefrigerant in the outlet of the heat source-side heat exchanger 4.Thus, the control unit 99 performs a control so that the ratio of thequantity of water flowing through the heat source water tubes 902, 903and the quantity of water flowing through the intermediate water tubes904, 905 is equal to the ratio of the heat source specific enthalpy andthe intermediate specific enthalpy. When the value of the heat sourcespecific enthalpy and/or the intermediate specific enthalpy changesduring adjustment of the flow rate ratio by control, the control unit 99may perform feedback control in accordance with predetermined timeintervals (or a predetermined degree of ratio deviation) so as to adaptto the ratio between the heat source specific enthalpy and theintermediate specific enthalpy at the point in time of the change.

Instead of merely performing control using the values of the heat sourcespecific enthalpy and/or the intermediate specific enthalpy in thismanner, the control unit 99 may control the ratio between the quantityof water flowing through the heat source water tubes 902, 903 and thequantity of water flowing through the intermediate water tubes 904, 905so that the outlet temperature of the heat source-side heat exchanger 4in the heat source water tube 903 and the outlet temperature of theintercooler 7 in the intermediate water tube 905 are substantiallyequal, for example. The control unit 99 may cause the outlettemperatures to coincide through a predetermined feedback control inthis case as well.

In cases in which the intention is to increase only the quantity ofwater in the heat source water tubes 902, 903, for example, it ispossible for the control unit 99 to also make adjustments for increasingthe quantity of water flowing through the heat source water tubes 902,903 while maintaining a constant quantity of water flowing through theintermediate water tubes 904, 905, by increasing the rotational speed ofthe motor of the pump 921 to increase the flow rate and by narrowing theopening degree of the flow rate ratio adjustment mechanism 911.

The control unit 99 can thereby perform control for bringing thecoefficient of performance of the refrigeration cycle to a satisfactoryvalue in an operation for achieving not only the user's desired outputwater temperature but the user's desired output water quantity as well.

<1-6> Modification 4

Another possibility, for example, is to use a refrigerant circuit 10Cwhich is provided with a heat source refrigerant temperature sensor 4Tfor detecting the temperature of the refrigerant passing through theheat source-side heat exchanger 4, a heat source refrigerant pressuresensor 4P for detecting the pressure of the refrigerant passing throughthe heat source-side heat exchanger 4, an intermediate refrigeranttemperature sensor 22T for detecting the temperature of the refrigerantpassing through the intermediate refrigerant tube 22, an intermediaterefrigerant pressure sensor 22P for detecting the pressure of therefrigerant passing through the intermediate refrigerant tube 22, andthe flow rate ratio adjustment mechanism 911 shown in Modification 3.

The radiated heat quantity that can be supplied to the water from therefrigerant in the heat source-side heat exchanger 4 can be perceivedaccording to the values sensed by the heat source refrigeranttemperature sensor 4T for detecting the temperature of the refrigerantpassing through the heat source-side heat exchanger 4 and the heatsource refrigerant pressure sensor 4P for detecting the pressure of therefrigerant passing through the heat source-side heat exchanger 4, andthe heat radiation quantity that can be supplied to the water from therefrigerant in the intercooler 7 can be perceived according to thevalues sensed by intermediate refrigerant temperature sensor 22T fordetecting the temperature of the refrigerant passing through theintermediate refrigerant tube 22 and the intermediate refrigerantpressure sensor 22P for detecting the pressure of the refrigerantpassing through the intermediate refrigerant tube 22. Therefore, thecontrol unit 99 can adjust the opening degree of the flow rate ratioadjustment mechanism 911 so as to adapt to the radiated heat quantitythat can be supplied to the water from the refrigerant in the heatsource-side heat exchanger 4 and the heat radiation quantity that can besupplied to the water from the refrigerant in the intercooler 7, and thecontrol unit 99 can perform control so as to achieve an efficient flowrate ratio for obtaining the required output water temperature.

<1-7> Modification 5

In Modification 4 described above, a water circuit 910 provided with aflow rate ratio adjustment mechanism 911 was described as an example.

However, the present invention is not limited to this example, and awater circuit may be used in which an on/off valve is provided to theheat source water tubes 902, 903 and an on/off valve is also provided tothe intermediate water tubes 904, 905, instead of the flow rate ratioadjustment mechanism 911, for example.

<1-8> Modification 6

In the embodiment described above, an example of a refrigerant circuitwas described in which only one two-stage compression mechanism wasprovided, wherein compression took place in two stages in the low-stagecompression element 2 c and the high-stage compression element 2 d.

However, the present invention is not limited to this example; anotherpossibility is to use a refrigerant circuit wherein the aforementionedtwo-stage compression mechanisms which perform compression in two stagesare provided in parallel to each other, for example.

In the refrigerant circuit, a plurality of usage-side heat exchangers 6may be disposed in parallel to each other. In this case, a refrigerantcircuit may be used in which expansion mechanisms are disposedimmediately ahead of the respective usage-side heat exchangers so thatthe quantity of refrigerant supplied to the usage-side heat exchangers 6can be controlled, and the expansion mechanisms are also disposed inparallel to each other.

<1-9> Modification 7

In the embodiment described above, an example was described in which thelow-stage compression element 2 c and the high-stage compression element2 d were provided with separate drive shafts 21 c, 21 f and compressordrive motors 21 b, 21 e.

However, the present invention is not limited to this example; anotherpossibility is a refrigerant circuit 10D which uses a compressionmechanism 2 having a shared drive shaft 121 c which is a drive shaftshared by the low-stage compression element 2 c and the high-stagecompression element 2 d, wherein one shared compressor drive motor 121 bis used to transmit drive force to the shared drive shaft 121 c, asshown in FIG. 7, for example.

This compression mechanism 2 has a hermetically sealed structure inwhich the compressor drive motor 121 b, the shared drive shaft 121 c,and the compression elements 2 c, 2 d are housed within a casing 21 a.The shared compressor drive motor 121 b is linked to the shared driveshaft 121 c. This shared drive shaft 121 c is linked to the twocompression elements 2 c, 2 d. That is, the compression mechanism has aso-called single-shaft two-stage compression structure in which the twocompression elements 2 c, 2 d are linked to a single shared drive shaft121 c, and the two compression elements 2 c, 2 d are both rotatablydriven by the shared compressor drive motor 121 b. The compressionelements 2 c, 2 d are rotary-type, scroll-type, or another type ofpositive displacement compression elements. The low-stage compressionelement 2 c draws refrigerant in from an intake tube 2 a, compresses thedrawn-in refrigerant, and discharges the refrigerant toward theintermediate refrigerant tube 22. The intermediate refrigerant tube 22connects the discharge side of the low-stage compression element 2 c andthe intake side of the high-stage compression element 2 d via theintercooler 7. The high-stage compression element 2 d further compressesthe refrigerant drawn in via the intermediate refrigerant tube 22 andthen discharges the refrigerant to the discharge tube 2 b. In FIG. 7,the discharge tube 2 b is a refrigerant tube for feeding the refrigerantdischarged from the compression mechanism 2 to the heat source-side heatexchanger 4, and the discharge tube 2 b is provided with an oilseparation mechanism 41 and a non-return mechanism 42. The oilseparation mechanism 41 is a mechanism for separating the refrigerantfrom refrigeration oil which accompanies the refrigerant discharged fromthe compression mechanism 2 and returning the refrigeration oil to theintake side of the compression mechanism 2, and the oil separationmechanism 41 has primarily an oil separator 41 a for separating therefrigerant from the refrigeration oil accompanying the refrigerantdischarged from the compression mechanism 2, and an oil return tube 41 bwhich is connected to the oil separator 41 a and which returns therefrigeration oil separated from the refrigerant to the intake tube 2 aof the compression mechanism 2. The oil return tube 41 b is providedwith a depressurization mechanism 41 c for depressurizing therefrigeration oil flowing through the oil return tube 41 b. A capillarytube is used as the depressurization mechanism 41 c. The non-returnmechanism 42 is a mechanism for allowing the flow of refrigerant fromthe discharge side of the compression mechanism 2 to the heatsource-side heat exchanger 4 and blocking the flow of refrigerant fromthe heat source-side heat exchanger 4 to the discharge side of thecompression mechanism 2, and a non-return valve is used.

Thus, the compression mechanism 2 has two compression elements 2 c, 2 d,and the compression mechanism 2 is configured so that refrigerantdischarged from the first-stage compression element of these compressionelements 2 c, 2 d is sequentially compressed by the second-stagecompression element.

Since a single-shaft two-stage compression mechanism is used herein, thecontrol unit 99 drives the low-stage compression element 2 c and thehigh-stage compression element 2 d while causing their centrifugalforces to cancel each other out to suppress vibrations and/orfluctuations in torque load, and the control unit 99 can perform controlso that the operating capacity of the low-stage compression element 2 cand the operating capacity of the high-stage compression element 2 d arebalanced, and the compression ratios are equal in the low-stage andhigh-stage elements.

<2> Second Embodiment <2-1> Configuration of Air-Conditioning Apparatus

FIG. 8 is a schematic structural diagram of a water heater 201 as arefrigeration apparatus according to the second embodiment of thepresent invention.

Components in the second embodiment having the same specifics as thoseof the first embodiment are not described hereinbelow.

(Water Circuit)

The water circuit 910 is the water circuit of the embodiment describedabove, but further having the flow rate ratio adjustment mechanism 911disposed at an intermediate point in the intermediate water tubes 904,905. The opening degree of this flow rate ratio adjustment mechanism 911is controlled by the control unit 99, and the ratio between the quantityof water flowing through the heat source water tubes 902, 903 and thequantity of water flowing through the intermediate water tubes 904, 905can be adjusted.

(Refrigerant Circuit)

The refrigerant circuit 210 is the refrigerant circuit of the embodimentdescribed above, but further having a liquid-gas heat exchanger 8, aliquid-gas three-way valve 8C, a liquid-gas bypass tube 8B, andconnecting tubes 71, 72, 73, 74, 75, 76, 77, and the like connectingthese components together.

The liquid-gas heat exchanger 8 has a liquid-side liquid-gas heatexchanger 8L through which passes the refrigerant flowing from theconnecting tube 73 to the connecting tube 74, and a gas-side liquid-gasheat exchanger 8G through which passes the refrigerant flowing from theconnecting tube 77 to the intake tube 2 a. The liquid-gas heat exchanger8 performs heat exchange between the refrigerant flowing through theliquid-side liquid-gas heat exchanger 8L and the refrigerant flowingthrough the gas-side liquid-gas heat exchanger 8G. Although thedescription uses wording such as “liquid”-side “liquid”-gas heatexchanger 8, the refrigerant passing through the liquid-side liquid-gasheat exchanger 8L is not limited to a liquid state, and may berefrigerant in a supercritical state, for example. Nor is therefrigerant flowing through the gas-side liquid-gas heat exchanger 8Glimited to refrigerant in a gas state, and refrigerant as moisture mayflow through, for example.

The liquid-gas bypass tube 8B connects one switching port of theliquid-gas three-way valve 8C connected to the connecting tube 73, whichis on the upstream side of the liquid-side liquid-gas heat exchanger 8L,and an end of the connecting tube 74 extending downstream of theliquid-side liquid-gas heat exchanger 8L.

The liquid-gas three-way valve 8C can switch between a liquid-gas usageconnection state in which the connection tube 72 extending from the heatsource-side heat exchanger 4 is connected to the connecting tube 73extending from the liquid-side liquid-gas heat exchanger 8L, and aliquid-gas non-usage connection state in which the connection tube 72extending from the heat source-side heat exchanger 4 is not connected tothe connecting tube 73 extending from the liquid-side liquid-gas heatexchanger 8L but is connected to the liquid-gas bypass tube 8B.

<2-2> Action of Air-Conditioning Apparatus

Next, the action of the water heater 201 of the second embodiment isdescribed using FIGS. 8, 9, and 10.

FIG. 9 is a pressure-enthalpy graph representing the refrigerationcycle, and FIG. 10 is a temperature-entropy graph representing therefrigeration cycle.

(Liquid-Gas Usage Connection State)

In the liquid-gas usage connection state, the connection state of theliquid-gas three-way valve 8C is switchably controlled by the controlunit 99 so that heat exchange is performed in the liquid-gas heatexchanger 8 between the refrigerant passing through the liquid-sideliquid-gas heat exchanger 8L and the refrigerant passing through thegas-side liquid-gas heat exchanger 8G.

Herein, the refrigerant drawn in from the intake tube 2 a of thelow-stage compression element 2 c (refer to point A in FIGS. 9 and 10)is compressed by the low-stage compression element 2 c (refer to point Bin FIGS. 9 and 10), and the refrigerant flowing through the intermediaterefrigerant tube 22 is cooled in the intercooler 7 by the water flowingthrough the intermediate water tubes 904, 905 (refer to point C in FIGS.9 and 10).

Having been compressed to a pressure exceeding the critical pressure bythe high-stage compression element 2 d (refer to point D in FIGS. 9 and10), the refrigerant is fed to the heat source-side heat exchanger 4. Inthe heat source-side heat exchanger 4, the water flowing through theheat source water tubes 902, 903 is then heated, whereby the heat withinthe refrigerant itself is radiated. Since carbon dioxide is used here asthe active refrigerant and the refrigerant flows into the heatsource-side heat exchanger 4 in a supercritical state, heat is radiatedto the exterior by the change in sensible heat while the refrigerantpressure remains constant in the heat radiation step, and thetemperature of the refrigerant itself continuously decreases (refer topoint K in FIGS. 9 and 10). Having exited the heat source-side heatexchanger 4, the refrigerant flows into the liquid-side liquid-gas heatexchanger 8L, where heat is further radiated due to heat exchange withthe low-temperature, low-pressure gas refrigerant flowing through thegas-side liquid-gas heat exchanger 8G, and the temperature of therefrigerant itself continuously decreases further (refer to point L inFIGS. 9 and 10). Having exited the liquid-side liquid-gas heat exchanger8L, the refrigerant is depressurized by the expansion mechanism 5 (referto point M in FIGS. 9 and 10), and the refrigerant flows into theusage-side heat exchanger 6. In the usage-side heat exchanger 6, due toheat exchange with external air and/or water while the pressure remainsconstant, the refrigerant evaporates while the heat taken from theexterior is consumed in a change of latent heat, whereby the dryness ofthe refrigerant increases (refer to point P in FIGS. 9 and 10). Havingexited the usage-side heat exchanger 6, the refrigerant evaporatesfurther while undergoing a change in latent heat in the gas-sideliquid-gas heat exchanger 8G due this time to the heat taken by heatexchange with the high-temperature, high-pressure refrigerant passingthrough the liquid-side liquid-gas heat exchanger 8L while the pressureremains constant, and the refrigerant reaches a superheated state abovethe dry saturated vapor curve at this pressure. This refrigerant in asuperheated state is then drawn into the low-stage compression element 2c through the intake tube 2 a (point A in FIGS. 9 and 10). Thisrefrigerant circulation is repeated in the liquid-gas usage connectionstate.

(Liquid-Gas Non-Usage Connection State)

In the liquid-gas non-usage connection state, the control unit 99controls the connection state of the liquid-gas three-way valve 8C andcreates a state in which the connection tube 72 and the liquid-gasbypass tube 8B are connected, so that heat exchange is not performed inthe liquid-gas heat exchanger 8.

The points A, B, C, D, and K of FIGS. 9 and 10 in the liquid-gasnon-usage connection state are the same as in the liquid-gas usageconnection state and are therefore not described.

The refrigerant that has exited the heat source-side heat exchanger 4herein flows through the liquid-gas bypass tube 8B to be depressurizedin the expansion mechanism 5 without flowing into the liquid-sideliquid-gas heat exchanger 8L (refer to point L′ in FIGS. 9 and 10). Therefrigerant is then depressurized in the expansion mechanism 5, and therefrigerant flows into the usage-side heat exchanger 6 (refer to pointM′ in FIGS. 9 and 10). In the usage-side heat exchanger 6, through heatexchange with external air and/or water while the pressure remainsconstant, the refrigerant evaporates while the heat taken from theexterior is consumed in a latent heat change, whereby the refrigerantreaches a superheated state above the dry saturated vapor curve at thispressure. This refrigerant in a superheated state is then drawn into thelow-stage compression element 2 c through the intake tube 2 a (refer topoint P′ in FIGS. 9 and 10). This refrigerant circulation is repeated inthe liquid-gas non-usage connection state.

(Liquid-Gas Heat Exchanger Switching Control)

The control unit 99 performs the same control as the target capacityoutput control described in Embodiment 1 above, and also performsliquid-gas heat exchanger switching control for switching between theabove-described liquid-gas usage connection state and liquid-gasnon-usage connection state.

In this liquid-gas heat exchanger switching control, the control unit 99switches the connection state of the liquid-gas three-way valve 8C inaccordance with the temperature sensed by the usage-side temperaturesensor 6T.

In the target capacity output control described above, the targetevaporation temperature is established based on the temperature sensedby the usage-side temperature sensor 6T, but when the temperature sensedby the usage-side temperature sensor 6T decreases and the targetevaporation temperature is set to be even lower, the dischargedrefrigerant temperature increases under the control condition that thetarget discharge pressure of the high-stage compression element 2 d doesnot change (under the condition that the required radiated heat quantitymust be guaranteed in the heat source-side heat exchanger 4). Thereliability of the high-stage compression element 2 d is compromisedwhen the discharged refrigerant temperature increases too much in thismanner. Therefore, the control unit 99 herein performs a control forbringing the connection state of the liquid-gas three-way valve 8C tothe liquid-gas non-usage connection state. Thereby, even if thetemperature sensed by the usage-side temperature sensor 6T decreases andthe target evaporation temperature is set to be even lower, the extentof the increase in the degree of superheating of the refrigerant drawnin by the high-stage compression element 2 d can be minimized tominimize the increase in the discharged refrigerant temperature, and therequired heat radiation quantity can be maintained.

On the other hand, in the target capacity output control describedabove, the target evaporation temperature is established based on thetemperature sensed by the usage-side temperature sensor 6T, but when thetemperature sensed by the usage-side temperature sensor 6T increases andthe target evaporation temperature is set to be even higher, thedischarged refrigerant temperature decreases under the control conditionthat the target discharge pressure of the high-stage compression element2 d does not change (under the condition that the required radiated heatquantity must be guaranteed in the heat source-side heat exchanger 4).In this case, it will sometimes not be possible to supply the heatsource-side heat exchanger 4 with refrigerant having the required heatradiation quantity. In such cases, the control unit 99 is capable ofswitching the connection state of the liquid-gas three-way valve 8C tothe liquid-gas usage connection state, raising the degree ofsuperheating of the refrigerant drawn into the high-stage compressionelement 2 d, and guaranteeing the heat radiation quantity required inthe heat source-side heat exchanger 4. There are also cases in which itis possible to improve the coefficient of performance even when therequired heat radiation quantity can be supplied in this manner. In suchcases, the control unit 99 is capable of guaranteeing the required heatradiation quantity and improving the coefficient of performance byswitching the connection state of the liquid-gas three-way valve 8C tothe liquid-gas usage connection state, lowering the specific enthalpy ofthe refrigerant drawn into the expansion mechanism 5, and improving therefrigerating capacity of the refrigeration cycle. Since the refrigerantdrawn into the low-stage compression element 2 c can be guaranteed tohave the appropriate degree of superheating, it is possible to preventthe risk of liquid compression occurring in the low-stage compressionelement 2 c.

<2-3> Modification 1

In the embodiment described above, an example was described in which thecontrol unit 99 switches the connection state of the liquid-gasthree-way valve 8C on the basis of the temperature sensed by theusage-side temperature sensor 6T (on the basis of the established targetevaporation temperature).

However, the present invention is not limited to this example; anotherpossibility is to use a refrigerant circuit 210A having a dischargedrefrigerant temperature sensor 2T for sensing the temperature of therefrigerant discharged from the high-stage compression element 2 d,instead of the usage-side temperature sensor 6T, as shown in FIG. 11,for example.

With this discharged refrigerant temperature sensor 2T, an increase inthe temperature sensed by the usage-side temperature sensor 6T describedabove corresponds to a decrease in the temperature sensed by thedischarged refrigerant temperature sensor 2T, and a decrease in thetemperature sensed by the usage-side temperature sensor 6T describedabove corresponds to an increase in the temperature sensed by thedischarged refrigerant temperature sensor 2T. That is, if thetemperature sensed by the discharged refrigerant temperature sensor 2Tis too high, it will not be possible to maintain the reliability of thehigh-stage compression element 2 d, and the control unit 99 willtherefore bring the connection state of the liquid-gas three-way valve8C to the liquid-gas non-usage connection state, preventing the degreeof superheating of the refrigerant drawn into the low-stage compressionelement 2 c from increasing. If the temperature sensed by the dischargedrefrigerant temperature sensor 2T decreases, it will not be possible tosupply the heat radiation quantity required in the heat source-side heatexchanger 4, and the control unit 99 therefore will bring the connectionstate of the liquid-gas three-way valve 8C to the liquid-gas usageconnection state and increase the degree of superheating of therefrigerant drawn into the low-stage compression element 2 c,guaranteeing a capacity. In conditions in which the refrigerant drawninto the low-stage compression element 2 c has a low temperature and thetemperature of the refrigerant discharged by the high-stage compressionelement 2 d does not increase excessively even if the degree ofsuperheating has been raised, the control unit 99 brings the connectionstate of the liquid-gas three-way valve 8C to the liquid-gas usageconnection state and lowers the specific enthalpy of the refrigerant fedto the expansion mechanism 5 to improve the refrigerating capacity ofthe refrigeration cycle, thereby raising the coefficient of performance.

<2-4> Modification 2

In the embodiment described above, an example was described in which theconnection state of the liquid-gas three-way valve 8C is switched toswitch between the liquid-gas usage connection state and the liquid-gasnon-usage connection state.

However, the present invention is not limited to this example; anotherpossibility is to cause refrigerant to flow to both the liquid-gasbypass tube 8B and the liquid-gas heat exchanger 8L and control therefrigerant flow rate ratio of both refrigerant passages by adjustingthe switched state of the liquid-gas three-way valve 8C, for example.

<2-5> Modification 3

In the embodiment described above, an example was described of arefrigerant circuit provided with the liquid-gas three-way valve 8C.

However, the present invention is not limited to this example; anotherpossibility is to use a refrigerant circuit which has an on/off valveprovided to the connecting tube 73 and an on/off valve also provided tothe liquid-gas bypass tube 8B, instead of the liquid-gas three-way valve8C, for example.

<2-6> Modification 4

In the embodiment described above, an example was described of arefrigerant circuit provided with only one two-stage compressionmechanism, wherein refrigerant is compressed in two stages in thelow-stage compression element 2 c and the high-stage compression element2 d.

However, the present invention is not limited to this example; anotherpossibility is to use a refrigerant circuit wherein the aforementionedtwo-stage compression mechanisms which perform compression in two stagesare provided in parallel to each other, for example.

In the refrigerant circuit, a plurality of usage-side heat exchangers 6may be disposed in parallel to each other. In this case, a refrigerantcircuit may be used in which expansion mechanisms are disposedimmediately ahead of the respective usage-side heat exchangers and theexpansion mechanisms are also disposed in parallel to each other so thatthe quantity of refrigerant supplied to the usage-side heat exchangers 6can be controlled.

<2-7> Modification 5

In the embodiment described above, an example was described in which thelow-stage compression element 2 c and the high-stage compression element2 d were provided with separate drive shafts 21 c, 21 f and compressordrive motors 21 b, 21 e.

However, the present invention is not limited to this example; anotherpossibility is a refrigerant circuit 210B which uses a compressionmechanism 2 having a shared drive shaft 121 c which is a drive shaftshared by the low-stage compression element 2 c and the high-stagecompression element 2 d, wherein one shared compressor drive motor 121 bis used to transmit drive force to the shared drive shaft 121 c, asshown in FIG. 12, for example.

This compression mechanism 2 has a hermetically sealed structure inwhich the compressor drive motor 121 b, the shared drive shaft 121 c,and the compression elements 2 c, 2 d are housed within a casing 21 a.The shared compressor drive motor 121 b is linked to the shared driveshaft 121 c. This shared drive shaft 121 c is linked to the twocompression elements 2 c, 2 d. That is, the compression mechanism has aso-called single-shaft two-stage compression structure in which the twocompression elements 2 c, 2 d are linked to a single shared drive shaft121 c, and the two compression elements 2 c, 2 d are both rotatablydriven by the shared compressor drive motor 121 b. The compressionelements 2 c, 2 d are rotary-type, scroll-type, or another type ofpositive displacement compression elements. The low-stage compressionelement 2 c draws refrigerant in from an intake tube 2 a, compresses thedrawn-in refrigerant, and discharges the refrigerant toward theintermediate refrigerant tube 22. The intermediate refrigerant tube 22connects the discharge side of the low-stage compression element 2 c andthe intake side of the high-stage compression element 2 d via theintercooler 7. The high-stage compression element 2 d further compressesthe refrigerant drawn in via the intermediate refrigerant tube 22 andthen discharges the refrigerant to the discharge tube 2 b. In FIG. 12,the discharge tube 2 b is a refrigerant tube for feeding the refrigerantdischarged from the compression mechanism 2 to the heat source-side heatexchanger 4, and the discharge tube 2 b is provided with an oilseparation mechanism 41 and a non-return mechanism 42. The oilseparation mechanism 41 is a mechanism for separating the refrigerantfrom refrigeration oil which accompanies the refrigerant discharged fromthe compression mechanism 2 and returning the refrigeration oil to theintake side of the compression mechanism 2, and the oil separationmechanism 41 has primarily an oil separator 41 a for separating therefrigerant from the refrigeration oil accompanying the refrigerantdischarged from the compression mechanism 2, and an oil return tube 41 bwhich is connected to the oil separator 41 a and which returns therefrigeration oil separated from the refrigerant to the intake tube 2 aof the compression mechanism 2. The oil return tube 41 b is providedwith a depressurization mechanism 41 c for depressurizing therefrigeration oil flowing through the oil return tube 41 b. A capillarytube is used as the depressurization mechanism 41 c. The non-returnmechanism 42 is a mechanism for allowing the flow of refrigerant fromthe discharge side of the compression mechanism 2 to the heatsource-side heat exchanger 4 and blocking the flow of refrigerant fromthe heat source-side heat exchanger 4 to the discharge side of thecompression mechanism 2, and a non-return valve is used.

Thus, the compression mechanism 2 has two compression elements 2 c, 2 d,and the compression mechanism 2 is configured so that refrigerantdischarged from the first-stage compression element of these compressionelements 2 c, 2 d is sequentially compressed by the second-stagecompression element.

Since a single-shaft two-stage compression mechanism is used herein, thecontrol unit 99 drives the low-stage compression element 2 c and thehigh-stage compression element 2 d while causing their centrifugalforces to cancel each other out to suppress vibrations and/orfluctuations in torque load, and the control unit 99 can perform controlso that the operating capacity of the low-stage compression element 2 cand the operating capacity of the high-stage compression element 2 d arebalanced, and the compression ratios are equal in the low-stage andhigh-stage elements.

<3> Third Embodiment <3-1> Configuration of Air-Conditioning Apparatus

FIG. 13 is a schematic structural diagram of a water heater 301, whichis a refrigeration apparatus according to the third embodiment of thepresent invention.

Components in the third embodiment that have the same specifications asthose of the first embodiment are not described hereinbelow.

(Water Circuit)

The water circuit 910 is the water circuit of the embodiment describedabove, but further having the flow rate ratio adjustment mechanism 911disposed at an intermediate point in the intermediate water tubes 904,905. The opening degree of this flow rate ratio adjustment mechanism 911is controlled by the control unit 99, and the ratio between the quantityof water flowing through the heat source water tubes 902, 903 and thequantity of water flowing through the intermediate water tubes 904, 905can be adjusted.

(Refrigerant Circuit)

The refrigerant circuit 310 is the refrigerant circuit of the embodimentdescribed above, but further having an economizer circuit 9, andconnecting tubes 73 c, 74 c, etc. connecting the other components.

The economizer circuit 9 has a branching upstream tube 9 a branching offfrom a branching point X between the connection tube 72 and theconnecting tube 73 c, an economizer expansion mechanism 9 e fordepressurizing refrigerant, a branching midstream tube 9 b for leadingthe refrigerant depressurized in the economizer expansion mechanism 9 eto the economizer heat exchanger 20, and a branching downstream tube 9 cfor leading the refrigerant that has flowed out of the economizer heatexchanger 20 to a convergent point Y in the intermediate refrigeranttube 22.

The connecting tube 73 c leads refrigerant through the economizer heatexchanger 20 to a connecting tube 75 c. This connecting tube 75 c isconnected to the expansion mechanism 5.

The configuration is otherwise the same as that of the water heater 1 ofthe first embodiment described above.

<3-2> Action of Air-Conditioning Apparatus

Next, the action of the water heater 301 of the third embodiment will bedescribed using FIGS. 13, 14, and 15.

FIG. 14 is a pressure-enthalpy graph representing the refrigerationcycle, and FIG. 15 is a temperature-entropy graph representing therefrigeration cycle.

(Economizer Usage State)

In an economizer usage state, refrigerant is caused to flow to theeconomizer circuit 9 by adjusting the opening degree of the economizerexpansion mechanism 9 e.

In the economizer circuit 9, refrigerant that has branched off from thebranching point X and flowed into the branching upstream tube 9 a isdepressurized in the economizer expansion mechanism 9 e (refer to pointR in FIGS. 13, 14, and 15), and the refrigerant flows into theeconomizer heat exchanger 20 via the branching midstream tube 9 b.

In the economizer heat exchanger 20, heat exchange takes place betweenthe refrigerant flowing through the connecting tube 73 c and theconnecting tube 75 c (refer to point X→point Q in FIGS. 13, 14, and 15),and the refrigerant flowing into the economizer heat exchanger 20 viathe branching midstream tube 9 b (refer to point R→point Y in FIGS. 13,14, and 15).

At this time, the refrigerant flowing through the connecting tube 73 cand the connecting tube 75 c is cooled by the refrigerant flowingthrough the branching midstream tube 9 b, which is being depressurizedand reduced in temperature in the economizer heat exchanger 20, and thespecific enthalpy decreases (refer to point X point Q in FIGS. 13, 14,and 15). Thus, the degree of supercooling of the refrigerant fed to theexpansion mechanism 5 increases, whereby the refrigeration capacity ofthe refrigeration cycle increases and the coefficient of performanceimproves. The refrigerant whose specific enthalpy has decreased isdepressurized by passing through the expansion mechanism 5, and therefrigerant flows into the usage-side heat exchanger 6 (refer to point Qpoint M in FIGS. 13, 14, and 15). The refrigerant evaporates in theusage-side heat exchanger 6, and the refrigerant is drawn into thelow-stage compression element 2 c (refer to point M point A in FIGS. 13,14, and 15). The refrigerant drawn into the low-stage compressionelement 2 c is compressed and increased in temperature, creating a statein which refrigerant increased in pressure to an intermediate pressureflows through the intermediate refrigerant tube 22.

When the refrigerant flowing through the intermediate refrigerant tube22 passes through the intercooler 7, the refrigerant radiates heat uponheating the water flowing through the intermediate water tubes 904, 905,and the refrigerant temperature decreases (refer to point B→point S inFIGS. 13, 14, and 15).

The refrigerant flowing into the economizer heat exchanger 20 via thebranching midstream tube 9 b is heated by the refrigerant flowingthrough the connecting tube 73 c and the connecting tube 75 c, wherebythe dryness of the refrigerant improves (refer to point R→point Y inFIGS. 13, 14, and 15).

Thus, refrigerant that has passed through the economizer circuit 9(refer to point Y in FIGS. 13, 14, an 15) mixes with refrigerant thathas been cooled in the intercooler 7 while flowing through theintermediate refrigerant tube 22 (refer to point S in FIGS. 13, 14, and15) at the convergent point Y in the intermediate refrigerant tube 22described above, the refrigerant temperature decreases while theintermediate pressure is maintained, the degree of superheating of therefrigerant discharged from the low-stage compression element 2 c isreduced, and the refrigerant is drawn into the high-stage compressionelement 2 d (refer to points Y, S, and C in FIGS. 13, 14, and 15). Thetemperature of the refrigerant drawn into the high-stage compressionelement 2 d thereby decreases, and it is therefore possible to preventthe temperature of the refrigerant discharged from the high-stagecompression element 2 d from increasing excessively. The refrigerantdensity also increases due to the temperature of the refrigerant drawninto the high-stage compression element 2 d decreasing, and the quantityof refrigerant circulating through the heat source-side heat exchanger 4is increased by the refrigerant injected via the economizer circuit 9;therefore, the capacity to supply refrigerant to the heat source-sideheat exchanger 4 can be greatly improved.

In the economizer usage state, this manner of refrigerant circulation isrepeated.

(Economizer Non-Usage State)

In the economizer non-usage state, the economizer expansion mechanism 9e in the economizer circuit 9 is brought to a fully closed state. Astate thereby arises in which the flow of refrigerant in the branchingmidstream tube 9 b ceases and the economizer heat exchanger 20 does notfunction (refer to points Q′, M′, and D′ in FIGS. 13, 14, and 15).

The cooling effects of the refrigerant flowing through the intermediaterefrigerant tube 22 thereby ceases, the temperature of the refrigerantdischarged from the high-stage compression element 2 d thereforeincreases, and it is possible to comply with a high output watertemperature requested by the user.

(Economizer Switching Control)

The control unit 99 performs the same control as the target capacityoutput control described in Embodiment 1 described above, and performseconomizer switching control for switching between the economizer usagestate and the economizer non-usage state described above.

In this economizer switching control, the control unit 99 controls theopening degree of the economizer expansion mechanism 9 e in accordancewith the temperature sensed by the usage-side temperature sensor 6T.

In the target capacity output control described above, the targetevaporation temperature is established based on the temperature sensedby the usage-side temperature sensor 6T, but when the temperature sensedby the usage-side temperature sensor 6T decreases and the targetevaporation temperature is set to be even lower, the dischargedrefrigerant temperature increases under the control condition that thetarget discharge pressure of the high-stage compression element 2 d doesnot change (under the condition that the required radiated heat quantitymust be guaranteed in the heat source-side heat exchanger 4). Thereliability of the high-stage compression element 2 d is compromisedwhen the discharged refrigerant temperature increases too much in thismanner. Therefore, the control unit 99 herein performs a control forcreating the economizer usage state, wherein the economizer heatexchanger 20 is made to function by opening the economizer expansionmechanism 9 e and causing refrigerant to flow to the economizer circuit9. Thereby, even if the temperature sensed by the usage-side temperaturesensor 6T decreases and the target evaporation temperature is set to beeven lower, the extent of the increase in the temperature of therefrigerant drawn in by the high-stage compression element 2 d can beminimized to minimize the increase in the discharged refrigeranttemperature, and the required heat radiation quantity can be maintained.

On the other hand, in the target capacity output control describedabove, the target evaporation temperature is established based on thetemperature sensed by the usage-side temperature sensor 6T, but when thetemperature sensed by the usage-side temperature sensor 6T increases andthe target evaporation temperature is set to be even higher, thedischarged refrigerant temperature decreases under the control conditionthat the target discharge pressure of the high-stage compression element2 d does not change (under the condition that the required radiated heatquantity must be guaranteed in the heat source-side heat exchanger 4).In this case, it will sometimes not be possible to supply the heatsource-side heat exchanger 4 with refrigerant having the required heatradiation quantity. In such cases, the control unit 99 is capable ofcreating the economizer non-usage state in which the economizerexpansion mechanism 9 e is fully closed, ensuring that the degree ofsuperheating of the refrigerant drawn in by the high-stage compressionelement 2 d does not decrease, and guaranteeing the heat radiationquantity required in the heat source-side heat exchanger 4. There arealso cases in which it is possible to improve the coefficient ofperformance even when the required heat radiation quantity can besupplied in this manner. In such cases, the control unit 99 is capableof guaranteeing the required heat radiation quantity and improving thecoefficient of performance by opening the economizer expansion mechanism9 e to create the economizer usage state, lowering the specific enthalpyof the refrigerant drawn into the expansion mechanism 5, and improvingthe refrigerating capacity of the refrigeration cycle.

<3-3> Modification 1

In the embodiment described above, an example was described in which thecontrol unit 99 switches the opening degree of the economizer expansionmechanism 9 e on the basis of the temperature sensed by the usage-sidetemperature sensor 6T (on the basis of the established targetevaporation temperature).

However, the present invention is not limited to this example; anotherpossibility is to use a refrigerant circuit 310A having a dischargedrefrigerant temperature sensor 2T for sensing the temperature of therefrigerant discharged from the high-stage compression element 2 d,instead of the usage-side temperature sensor 6T, as shown in FIG. 16,for example.

With this discharged refrigerant temperature sensor 2T, an increase inthe temperature sensed by the usage-side temperature sensor 6T describedabove corresponds to a decrease in the temperature sensed by thedischarged refrigerant temperature sensor 2T, and a decrease in thetemperature sensed by the usage-side temperature sensor 6T describedabove corresponds to an increase in the temperature sensed by thedischarged refrigerant temperature sensor 2T. That is, if thetemperature sensed by the discharged refrigerant temperature sensor 2Tis too high, it will not be possible to maintain the reliability of, thehigh-stage compression element 2 d, and the control unit 99 willtherefore increase the opening degree of the economizer expansionmechanism 9 e to bring about the economizer usage state, preventing thedischarged refrigerant temperature of the high-stage compression element2 d from increasing excessively by reducing the degree of superheatingof the refrigerant drawn into the low-stage compression element 2 c. Ifthe temperature sensed by the discharged refrigerant temperature sensor2T decreases, it will not be possible to supply the heat radiationquantity required in the heat source-side heat exchanger 4, and thecontrol unit 99 therefore will fully close the economizer expansionmechanism 9 e to bring about the economizer non-usage state andguarantee a capacity without reducing the degree of superheating of therefrigerant drawn into the high-stage compression element 2 d. Inconditions in which the refrigerant drawn into the high-stagecompression element 2 d has a low temperature and the temperature of therefrigerant discharged by the high-stage compression element 2 d doesnot increase excessively even if the degree of superheating has beenraised, the control unit 99 increases the opening degree of theeconomizer expansion mechanism 9 e to bring about the economizer usagestate and lowers the specific enthalpy of the refrigerant fed to theexpansion mechanism 5 to improve the refrigerating capacity of therefrigeration cycle, thereby raising the coefficient of performance.

<3-4> Modification 2

In the embodiment described above, an example was described in which theopening degree of the economizer expansion mechanism 9 e is adjusted toswitch between the economizer usage state and the economizer non-usagestate.

However, the present invention is not limited to this example; anotherpossibility is to control the flow rate ratio of the refrigerantsflowing to the economizer circuit 9 and the connecting tubes 73 c, 75 cby adjusting the valve opening degree of the economizer expansionmechanism 9 e, for example.

<3-5> Modification 3

In the embodiment described above, an example was described of arefrigerant circuit provided with only one two-stage compressionmechanism, wherein refrigerant is compressed in two stages in thelow-stage compression element 2 c and the high-stage compression element2 d.

However, the present invention is not limited to this example; anotherpossibility is to use a refrigerant circuit wherein the aforementionedtwo-stage compression mechanisms which perform compression in two stagesare provided in parallel to each other, for example.

In the refrigerant circuit, a plurality of usage-side heat exchangers 6may be disposed in parallel to each other. In this case, a refrigerantcircuit may be used in which expansion mechanisms are disposedimmediately ahead of the respective usage-side heat exchangers and theexpansion mechanisms are also disposed in parallel to each other so thatthe quantity of refrigerant supplied to the usage-side heat exchangers 6can be controlled.

<3-6> Modification 4

In the embodiment described above, an example was described in which thelow-stage compression element 2 c and the high-stage compression element2 d were provided with the separate drive shafts 21 c, 21 f and thecompressor drive motors 21 b, 21 e.

However, the present invention is not limited to this example; anotherpossibility is a refrigerant circuit 310B which uses a compressionmechanism 2 having a shared drive shaft 121 c which is a drive shaftshared by the low-stage compression element 2 c and the high-stagecompression element 2 d, wherein one shared compressor drive motor 121 bis used to transmit drive force to the shared drive shaft 121 c, asshown in FIG. 17, for example.

This compression mechanism 2 has a hermetically sealed structure inwhich the compressor drive motor 121 b, the shared drive shaft 121 c,and the compression elements 2 c, 2 d are housed within a casing 21 a.The shared compressor drive motor 121 b is linked to the shared driveshaft 121 c. This shared drive shaft 121 c is linked to the twocompression elements 2 c, 2 d. That is, the compression mechanism has aso-called single-shaft two-stage compression structure in which the twocompression elements 2 c, 2 d are linked to a single shared drive shaft121 c, and the two compression elements 2 c, 2 d are both rotatablydriven by the shared compressor drive motor 121 b. The compressionelements 2 c, 2 d are rotary-type, scroll-type, or another type ofpositive displacement compression elements. The low-stage compressionelement 2 c draws refrigerant in from an intake tube 2 a, compresses thedrawn-in refrigerant, and discharges the refrigerant toward theintermediate refrigerant tube 22. The intermediate refrigerant tube 22connects the discharge side of the low-stage compression element 2 c andthe intake side of the high-stage compression element 2 d via theintercooler 7. The high-stage compression element 2 d further compressesthe refrigerant drawn in via the intermediate refrigerant tube 22 andthen discharges the refrigerant to the discharge tube 2 b. In FIG. 17,the discharge tube 2 b is a refrigerant tube for feeding the refrigerantdischarged from the compression mechanism 2 to the heat source-side heatexchanger 4, and the discharge tube 2 b is provided with an oilseparation mechanism 41 and a non-return mechanism 42. The oilseparation mechanism 41 is a mechanism for separating the refrigerantfrom refrigeration oil which accompanies the refrigerant discharged fromthe compression mechanism 2 and returning the refrigeration oil to theintake side of the compression mechanism 2, and the oil separationmechanism 41 has primarily an oil separator 41 a for separating therefrigerant from the refrigeration oil accompanying the refrigerantdischarged from the compression mechanism 2, and an oil return tube 41 bwhich is connected to the oil separator 41 a and which returns therefrigeration oil separated from the refrigerant to the intake tube 2 aof the compression mechanism 2. The oil return tube 41 b is providedwith a depressurization mechanism 41 c for depressurizing therefrigeration oil flowing through the oil return tube 41 b. A capillarytube is used as the depressurization mechanism 41 c. The non-returnmechanism 42 is a mechanism for allowing the flow of refrigerant fromthe discharge side of the compression mechanism 2 to the heatsource-side heat exchanger 4 and blocking the flow of refrigerant fromthe heat source-side heat exchanger 4 to the discharge side of thecompression mechanism 2, and a non-return valve is used.

Thus, the compression mechanism 2 has the two compression elements 2 c,2 d, and the compression mechanism 2 is configured so that refrigerantdischarged from the first-stage compression element of these compressionelements 2 c, 2 d is sequentially compressed by the second-stagecompression element.

Since a single-shaft two-stage compression mechanism is used herein, thecontrol unit 99 drives the low-stage compression element 2 c and thehigh-stage compression element 2 d while causing their centrifugalforces to cancel each other out to suppress vibrations and/orfluctuations in torque load, and the control unit 99 can perform controlso that the operating capacity of the low-stage compression element 2 cand the operating capacity of the high-stage compression element 2 d arebalanced, and the compression ratios are equal in the low-stage andhigh-stage elements.

<4> Fourth Embodiment <4-1> Configuration of Air-Conditioning Apparatus

FIG. 18 is a schematic structural diagram of a water heater 401, whichis a refrigeration apparatus according to the fourth embodiment of thepresent invention.

Components in the fourth embodiment that have the same specifics asthose of the first embodiment are not described herein.

(Water Circuit)

The water circuit 910 is the water circuit of the embodiment describedabove, but further having the flow rate ratio adjustment mechanism 911disposed at an intermediate point in the intermediate water tubes 904,905. The opening degree of this flow rate ratio adjustment mechanism 911is controlled by the control unit 99, and the ratio between the quantityof water flowing through the heat source water tubes 902, 903 and thequantity of water flowing through the intermediate water tubes 904, 905can be adjusted.

(Refrigerant Circuit)

The refrigerant circuit 410 is the refrigerant circuit of the embodimentdescribed above, but further having the liquid-gas heat exchanger 8, aswitching three-way valve 28C, the liquid-gas bypass tube 8B, theeconomizer circuit 9, and connecting tubes 74 g, 75 g, etc. connectingthese components.

The liquid-gas heat exchanger 8 has a liquid-side liquid-gas heatexchanger 8L through which passes the refrigerant flowing from theconnecting tube 73 to the connecting tube 74, and a gas-side liquid-gasheat exchanger 8G through which passes the refrigerant flowing from theconnecting tube 77 to the intake tube 2 a. The liquid-gas heat exchanger8 performs heat exchange between the refrigerant flowing through theliquid-side liquid-gas heat exchanger 8L and the refrigerant flowingthrough the gas-side liquid-gas heat exchanger 8G. Although thedescription uses wording such as “liquid”-side “liquid”-gas heatexchanger 8, the refrigerant passing through the liquid-side liquid-gasheat exchanger 8L is not limited to a liquid state, and may berefrigerant in a supercritical state, for example. Nor is therefrigerant flowing through the gas-side liquid-gas heat exchanger 8Glimited to refrigerant in a gas state, and refrigerant as moisture mayflow through, for example. An expansion mechanism 95 e is provided at anintermediate point in the connecting tube 74.

The liquid-gas bypass tube 8B connects one switching port of theswitching three-way valve 28C connected to the connecting tube 73, whichis on the upstream side of the liquid-side liquid-gas heat exchanger 8L,and an end of the connecting tube 74 extending downstream of theliquid-side liquid-gas heat exchanger 8L.

The switching three-way valve 28C can switch between a liquid-gas statein which the connection tube 72 extending from the heat source-side heatexchanger 4 is connected to the connecting tube 73 extending from theliquid-side liquid-gas heat exchanger 8L, and an economizer state inwhich the connection tube 72 extending from the heat source-side heatexchanger 4 is not connected to the connecting tube 73 extending fromthe liquid-side liquid-gas heat exchanger 8L but is connected to theliquid-gas bypass tube 8B. By fully closing the economizer expansionmechanism 9 e in the economizer state, a dual-function non-usage statecan be brought about in which neither the economizer circuit 9 nor theliquid-gas heat exchanger 8 are used.

The economizer circuit 9 has a branching upstream tube 9 a branching offfrom a branching point X between the liquid-gas bypass tube 8B and theconnecting tube 74 g, an economizer expansion mechanism 9 e fordepressurizing refrigerant, a branching midstream tube 9 b for leadingthe refrigerant depressurized in the economizer expansion mechanism 9 eto the economizer heat exchanger 20, and a branching downstream tube 9 cfor leading the refrigerant that has flowed out of the economizer heatexchanger 20 to a convergent point Y in the intermediate refrigeranttube 22.

The connecting tube 74 g leads refrigerant through the economizer heatexchanger 20 to a connecting tube 75 g. This connecting tube 75 g isconnected to the expansion mechanism 5.

The refrigerant that has passed through the liquid-side liquid-gas heatexchanger 8L and the refrigerant that has passed through the economizerheat exchanger 20 mix together at a convergent point L and flow into theusage-side heat exchanger 6 via the connection tube 76.

The control unit 99 can switch between the economizer state, theliquid-gas state, and the dual-function non-usage stage by adjusting theconnection state of the switching three-way valve 28C and the openingdegree of the economizer expansion mechanism 9 e.

The configuration otherwise has the same specifics as those described inthe above-described water heater 1 of the first embodiment, the waterheater 201 of the second embodiment, and/or the water heater 301 of thethird embodiment.

<4-2> Action of the Air-Conditioning Apparatus

Next, the action of the water heater 401 of the fourth embodiment willbe described using FIGS. 18, 19, and 20.

FIG. 19 is a pressure-enthalpy graph representing the refrigerationcycle, and FIG. 20 is a temperature-entropy graph representing therefrigeration cycle.

Between the specific enthalpy at point Q in the economizer state and thespecific enthalpy at point T in the liquid-gas state, whichever has thegreater value changes depending on the opening degree of the expansionmechanism 5 and/or the expansion mechanism 95 e, and these specificenthalpies are therefore not limited to the examples shown in FIGS. 19and 20.

(Economizer State)

In the economizer state, the control unit 99 switches the connectionstate of the switching three-way valve 28C so that refrigerant does notflow to the connecting tube 73 but refrigerant does flow to theliquid-gas bypass tube 8B, increases the opening degree of theeconomizer expansion mechanism 9 e, and performs the refrigeration cycleso that the refrigerant flows to the economizer circuit 9. The samerefrigeration cycle as in the economizer usage state in Embodiment 3described above is performed here, as shown by points A, B, C, D, K, X,R, Y, Q, L, and P in FIGS. 18, 19, and 20.

The specific enthalpy of the refrigerant flowing through the connectingtube 75 g into the expansion mechanism 5 herein can be lowered by heatexchange in the economizer heat exchanger 20, and the refrigeratingcapacity of the refrigeration cycle can be improved to bring thecoefficient of performance to a satisfactory value. Furthermore, thedegree of superheating of the refrigerant drawn into the high-stagecompression element 2 d can be further reduced by the refrigerantpassing through the economizer circuit 9 and mixing at the convergentpoint Y of the intermediate refrigerant tube 22, rather than by theintercooler 7 alone, the density of the refrigerant drawn into thecompression element 2 d can be raised to improve compression efficiency,and abnormal increases in the discharged refrigerant temperature can beprevented. At this time, injection into the intermediate refrigeranttube 22 via the economizer circuit 9 makes it possible to increase thequantity of refrigerant supplied to the heat source-side heat exchanger4 and to increase the quantity of heat supplied as well.

(Liquid-Gas State)

In the liquid-gas state, the control unit 99 switches the connectionstate of the switching three-way valve 28C so that refrigerant does notflow to the liquid-gas bypass tube 8B but does flow to the connectingtube 73, and performs a refrigeration cycle that makes the liquid-gasheat exchanger 8 function. The same refrigeration cycle as in theliquid-gas usage connection state in Embodiment 2 described above isperformed here, as shown by points A, B, C′, D, K, T, L′, and P′ inFIGS. 18, 19, and 20.

The specific enthalpy of the refrigerant flowing into the expansionmechanism 95 e here can be lowered; therefore, the refrigeratingcapacity in the refrigeration cycle can be improved to bring thecoefficient of performance to a satisfactory value, a degree ofsuperheating can be guaranteed in the refrigerant drawn into thelow-stage compression element 2 c to prevent liquid compression, and thedischarge temperature can be increased to guarantee the heat quantityrequired in the heat source-side heat exchanger 4.

(Dual-Function Non-Usage State)

In the dual-function non-usage state, the control unit 99 switches theconnection state of the switching three-way valve 28C so thatrefrigerant does not flow to the connecting tube 73 but does flow to theliquid-gas bypass tube 8B, fully closes the economizer expansionmechanism 9 e, and performs the refrigeration cycle so that neither theeconomizer circuit 9 nor the liquid-gas heat exchanger 8 are used. Asimple refrigeration cycle is performed here, as shown by points A, B,C, D″, K, X, Q″, L″, and P in FIGS. 18, 19, and 20.

Since the temperature of the refrigerant discharged from the high-stagecompression element 2 d can be increased here, it is possible to supplythe required heat quantity even in cases in which the radiated heatquantity required in the heat source-side heat exchanger 4 hasincreased.

(Switching Control of Economizer State, Liquid-Gas State, andDual-Function Non-Usage State)

The control unit 99 performs a control for switching the above-describedstates such that the first priority is that the discharged refrigeranttemperature be within a range of not increasing abnormally and thedischarged refrigerant pressure be a pressure equal to or less than thepressure capacity of the low-stage compression element 2 c and thehigh-stage compression element 2 d, the second priority is that theoutput water temperature and output water quantity requested by the userbe achieved, and the third priority is that the operating efficiency besatisfactory (that an appropriate balance can be established betweenimproving the coefficient of performance and increasing compressionefficiency).

That is, in cases in which the radiated heat quantity of the refrigerantin the heat source-side heat exchanger 4 is insufficient, control isperformed such that the liquid-gas state goes into effect if thedischarge temperature is within a range of not increasing abnormally,and the dual-function non-usage state goes into effect if an abnormalincrease in the discharge temperature is avoided. In cases in which theheat radiation quantity in the heat source-side heat exchanger 4 issufficient, the economizer state goes into effect, the opening degree ofthe economizer expansion mechanism 9 e is controlled, the valve openingdegree is increased within a margin whereby the heat radiation quantityrequired in the heat source-side heat exchanger 4 can be supplied, thecoefficient of performance is brought to a satisfactory value byimproving the refrigerating capacity of the refrigeration cycle, andcontrol is performed for increasing the supplied heat quantity byincreasing the quantity of refrigerant that can be supplied to the heatsource-side heat exchanger 4.

The heat radiation quantity herein is determined by the control unit 99on the basis of the temperature sensed by the water temperature sensor910T, as well as the output water temperature and the output waterquantity required by the user. Whether or not the discharge temperaturehas increased abnormally is determined by the control unit 99 on thebasis of (the evaporation temperature established corresponding to) thetemperature sensed by the usage-side temperature sensor 6T.

<4-3> Modification 1

In the embodiment described above, an example was described of a case inwhich the control unit 99 performs a control for switching between theeconomizer state, the liquid-gas state, and the dual-function non-usagestate.

However, the present invention is not limited to this example; anotherpossibility is to allow the use of a dual-usage state in which theeconomizer circuit 9 is used while the liquid-gas heat exchanger 8 isused as well, for example.

On the preconditions that the discharged refrigerant temperature of thehigh-stage compression element 2 d be within a range of not increasingabnormally, the discharged refrigerant pressure be one equal to or lessthan the pressure capacity of the low-stage compression element 2 c andthe high-stage compression element 2 d, and it be possible to supply theoutput water temperature and output water quantity requested by theuser, for example, the control unit 99 herein may control the ratiobetween the flow rate of refrigerant flowing through the economizercircuit 9 and the flow rate of the liquid-gas heat exchanger 8L whilerefrigerant is flowing simultaneously to both the economizer circuit 9and the liquid-gas heat exchanger 8L, rather than simply alternatelyswitching the connection state of the switching three-way valve 28C, sothat the operating efficiency can be made satisfactory (an appropriatebalance can be established between improving the coefficient ofperformance and increasing compression efficiency). The configurationwhich can adjust the ratio herein is not limited to the switchingthree-way valve 28C, and an expansion mechanism may be providedimmediately ahead of the liquid-gas heat exchanger 8L to perform flowrate ratio control, for example.

For the ratio between the flow rate in the economizer circuit 9 and theflow rate in the liquid-gas heat exchanger 8, the control unit 99 hereinensures that the discharged refrigerant temperature of the high-stagecompression element 2 d is within a range of not increasing abnormally(under conditions such as the temperature of the refrigerant dischargedfrom the high-stage compression element 2 d being equal to or less thana predetermined temperature) when the target evaporation temperature isestablished based on the temperature sensed by the usage-sidetemperature sensor 6T, and also that the discharged refrigerant pressureis equal to or less than the pressure capacity of the low-stagecompression element 2 c and the high-stage compression element 2 d; andthe control unit 99 calculates a heat quantity sufficient to guaranteethe output water temperature and output water quantity requested by theuser.

Assuming that the flow rate of the economizer circuit 9 is zero, forexample, the control unit 99 then calculates a flow rate of theliquid-gas heat exchanger 8L needed in order to guarantee a radiatedheat quantity, whereby abnormal increases in the discharged refrigeranttemperature at the target evaporation temperature can be prevented, andthe discharge pressure is equal to or less than a predetermined pressurecorresponding to the pressure capacity of the low-stage compressionelement 2 c and the high-stage compression element 2 d. Next, whilereducing this calculated flow rate in the liquid-gas heat exchanger 8Land assuming that refrigerant equivalent to the reduced flow rate hasflowed to the economizer circuit 9, the control unit 99 controls theflow rate ratio so that the respective compression ratios of thelow-stage compression element 2 c and the high-stage compression element2 d are within a predetermined range and the coefficient of performanceis within a predetermined range, while taking into account the decreasein the refrigerating capacity resulting from the increase in specificenthalpy accompanying the decrease in the flow rate of the liquid-gasheat exchanger 8, the increase in the refrigerating capacity resultingfrom the decrease in specific enthalpy accompanying the increase in theflow rate of the economizer circuit 9, the increase in the compressionratio of the compression mechanism resulting from the high pressureincreasing in order to guarantee the heat radiation quantity byincreasing the flow rate of the economizer circuit 9, and the increasein the supplied heat amount accompanying the increase in the refrigerantdensity supplied to the heat source-side heat exchanger 4 resulting fromthe increase in the flow rate of the economizer circuit 9.

For example, in this flow rate control by the control unit 99, anintermediate pressure at which the compression ratio of the low-stagecompression element 2 c and the compression ratio of the high-stagecompression element 2 d are equal may be calculated as the intermediatepressure for minimizing the compression work, and the economizerexpansion mechanism 9 e may be controlled so that the extent ofdepressurization in the economizer expansion mechanism 9 e yields thisintermediate pressure (and pressures within a predetermined range fromthis intermediate pressure), whereupon the flow rate ratio in theswitching three-way valve 28C may be adjusted so that the coefficient ofperformance is satisfactory.

<4-4> Modification 2

In the embodiment described above, an example was described in which thecontrol unit 99 switches the opening degree of the switching three-wayvalve 28C and/or the economizer expansion mechanism 9 e on the basis ofthe temperature sensed by the usage-side temperature sensor 6T (on thebasis of the established target evaporation temperature).

However, the present invention is not limited to this example; anotherpossibility is to use a refrigerant circuit 410A having a dischargedrefrigerant temperature sensor 2T for sensing the temperature of therefrigerant discharged from the high-stage compression element 2 d,instead of the usage-side temperature sensor 6T, as shown in FIG. 21,for example.

With this discharged refrigerant temperature sensor 2T, an increase inthe temperature sensed by the usage-side temperature sensor 6T describedabove corresponds to a decrease in the temperature sensed by thedischarged refrigerant temperature sensor 2T, and a decrease in thetemperature sensed by the usage-side temperature sensor 6T describedabove corresponds to an increase in the temperature sensed by thedischarged refrigerant temperature sensor 2T.

<4-5> Modification 3

In the embodiment described above, an example was described of a case inwhich the connection state of the switching three-way valve 28C isswitched to switch between the liquid-gas state, the economizer state,and the dual-function non-usage state.

However, the present invention is not limited to this example; anotherpossibility is to use a refrigerant circuit which has an on/off valveprovided to the connecting tube 73 g and an on/off valve also providedto the connecting tube 73, instead of the switching three-way valve 28C,for example.

<4-6> Modification 4

In the embodiment described above, an example was described of arefrigerant circuit 410 provided with both an expansion mechanism 5 andan expansion mechanism 95 e.

However, the present invention is not limited to this example; anotherpossibility is to use a refrigerant circuit 410B having a dual-useexpansion mechanism 405B which can be used both in control during theeconomizer state and control during the liquid-gas state, as shown inFIG. 22, for example. This dual-use expansion mechanism 405B is providedat an intermediate point in a connecting tube 76 g extending from theconvergent point L to the usage-side heat exchanger 6.

In this case, the number of expansion mechanisms can be reduced to fewerthan that of the refrigerant circuit 410 in the fourth embodimentdescribed above.

<4-7> Modification 5

In the embodiment described above, an example was described of arefrigerant circuit 410 in which the branching point X which branches inthe economizer circuit 9 is bypassed by the liquid-gas heat exchanger 8.

However, the present invention is not limited to this example; anotherpossibility is to use a refrigerant circuit 410C in which the branchingpoint X which branches in the economizer circuit 9 is provided between adual-use expansion mechanism 405C and a convergent point V between theconnecting tube 74 and the liquid-gas bypass tube 8B which extends fromthe liquid-gas three-way valve 8C for switching between the liquid-gasstate and the economizer state, as shown in FIG. 23, for example.

<4-8> Modification 6

Furthermore, another possibility is to use a refrigerant circuit 410D inwhich the branching point X which branches in the economizer circuit 9is provided between the liquid-gas three-way valve 8C and the heatsource-side heat exchanger 4, as shown in FIG. 24.

In this refrigerant circuit 410D, the connection in the liquid-gasthree-way valve 8C is switched between leading to the connecting tube 73and leading to the liquid-gas bypass tube 8B. The refrigerant that haspassed through the liquid-gas heat exchanger 8L then passes through theconnecting tube 74 and mixes at the convergent point L with theliquid-gas bypass tube 8B. The connecting tube 76 g and a dual-useexpansion mechanism 405D are provided between this convergent point Land the usage-side heat exchanger 6.

<4-9> Modification 7

In the embodiment described above, an example was described of arefrigerant circuit provided with only one two-stage compressionmechanism, wherein refrigerant is compressed in two stages in thelow-stage compression element 2 c and the high-stage compression element2 d.

However, the present invention is not limited to this example; anotherpossibility is to use a refrigerant circuit wherein the above-describedtwo-stage compression mechanisms which perform compression in two stagesare provided in parallel to each other, for example.

In the refrigerant circuit, a plurality of usage-side heat exchangers 6may be disposed in parallel to each other. In this case, a refrigerantcircuit may be used in which expansion mechanisms are disposedimmediately ahead of the respective usage-side heat exchangers and theexpansion mechanisms are also disposed in parallel to each other so thatthe quantity of refrigerant supplied to the usage-side heat exchangers 6can be controlled.

<4-10> Modification 8

In the embodiment described above, an example was described in which thelow-stage compression element 2 c and the high-stage compression element2 d were provided with the separate drive shafts 21 c, 21 f and thecompressor drive motors 21 b, 21 e.

However, the present invention is not limited to this example; anotherpossibility is a refrigerant circuit 410E which uses a compressionmechanism 2 having a shared drive shaft 121 c which is a drive shaftshared by the low-stage compression element 2 c and the high-stagecompression element 2 d, wherein one shared compressor drive motor 121 bis used to transmit drive force to the shared drive shaft 121 c, asshown in FIG. 25, for example.

This compression mechanism 2 has a hermetically sealed structure inwhich the compressor drive motor 121 b, the shared drive shaft 121 c,and the compression elements 2 c, 2 d are housed within a casing 21 a.The shared compressor drive motor 121 b is linked to the shared driveshaft 121 c. This shared drive shaft 121 c is linked to the twocompression elements 2 c, 2 d. That is, the compression mechanism has aso-called single-shaft two-stage compression structure in which the twocompression elements 2 c, 2 d are linked to a single shared drive shaft121 c, and the two compression elements 2 c, 2 d are both rotatablydriven by the shared compressor drive motor 121 b. The compressionelements 2 c, 2 d are rotary-type, scroll-type, or another type ofpositive displacement compression elements. The low-stage compressionelement 2 c draws refrigerant in from an intake tube 2 a, compresses thedrawn-in refrigerant, and discharges the refrigerant toward theintermediate refrigerant tube 22. The intermediate refrigerant tube 22connects the discharge side of the low-stage compression element 2 c andthe intake side of the high-stage compression element 2 d via theintercooler 7. The high-stage compression element 2 d further compressesthe refrigerant drawn in via the intermediate refrigerant tube 22 andthen discharges the refrigerant to the discharge tube 2 b. In FIG. 25,the discharge tube 2 b is a refrigerant tube for feeding the refrigerantdischarged from the compression mechanism 2 to the heat source-side heatexchanger 4, and the discharge tube 2 b is provided with an oilseparation mechanism 41 and a non-return mechanism 42. The oilseparation mechanism 41 is a mechanism for separating the refrigerantfrom refrigeration oil which accompanies the refrigerant discharged fromthe compression mechanism 2 and returning the refrigeration oil to theintake side of the compression mechanism 2, and the oil separationmechanism 41 has primarily an oil separator 41 a for separating therefrigerant from the refrigeration oil accompanying the refrigerantdischarged from the compression mechanism 2, and an oil return tube 41 bwhich is connected to the oil separator 41 a and which returns therefrigeration oil separated from the refrigerant to the intake tube 2 aof the compression mechanism 2. The oil return tube 41 b is providedwith a depressurization mechanism 41 c for depressurizing therefrigeration oil flowing through the oil return tube 41 b. A capillarytube is used as the depressurization mechanism 41 c. The non-returnmechanism 42 is a mechanism for allowing the flow of refrigerant fromthe discharge side of the compression mechanism 2 to the heatsource-side heat exchanger 4 and blocking the flow of refrigerant fromthe heat source-side heat exchanger 4 to the discharge side of thecompression mechanism 2, and a non-return valve is used.

Thus, the compression mechanism 2 has two compression elements 2 c, 2 d,and the compression mechanism 2 is configured so that refrigerantdischarged from the first-stage compression element of these compressionelements 2 c, 2 d is sequentially compressed by the second-stagecompression element.

Since a single-shaft two-stage compression mechanism is used herein, thecontrol unit 99 drives the low-stage compression element 2 c and thehigh-stage compression element 2 d while causing their centrifugalforces to cancel each other out to suppress vibrations and/orfluctuations in torque load, and the control unit 99 can perform controlso that the operating capacity of the low-stage compression element 2 cand the operating capacity of the high-stage compression element 2 d arebalanced, and the compression ratios are equal in the low-stage andhigh-stage elements.

<5> Other Embodiments

Embodiments of the present invention and modifications thereof weredescribed above based on the drawings, but the specific configuration isnot limited to these embodiments and their modifications; other changescan be made within a range that does not deviate from the scope of theinvention.

For example, in the embodiments and their modifications described above,the present invention may be applied to a so-called chiller-typeair-conditioning apparatus in which water and/or brine is used as theheating source or cooling source for performing heat exchange with therefrigerant flowing through the usage-side heat exchanger 6, and asecondary heat exchanger is provided for performing heat exchangebetween indoor air and the water and/or brine that has undergone heatexchange in the usage-side heat exchanger 6.

The present invention can also be applied even to a refrigerationapparatus of a type different from the aforementioned chiller-typeair-conditioning apparatus, such as a cooling-only air-conditioningapparatus or the like.

The refrigerant that operates in a supercritical range is not limited tocarbon dioxide; ethylene, ethane, nitric oxide, and the like may also beused.

INDUSTRIAL APPLICABILITY

In the refrigeration apparatus of the present invention, sincecompression efficiency can be more reliably improved in therefrigeration apparatus and the heating of the water for the hot watersupply can be made more efficient, the refrigeration apparatus isparticularly useful in cases in which the present invention is appliedto a refrigeration apparatus which has multi-stage compression-typecompression elements and which uses refrigerant that operates includingthe supercritical state process as the active refrigerant.

REFERENCE SIGNS LIST

-   1 Air-conditioning apparatus (refrigeration apparatus)-   2 Compression mechanism-   4 Heat source-side heat exchanger-   5 Expansion mechanism-   6 Usage-side heat exchanger-   7 Intercooler-   8 Liquid-gas heat exchanger-   10 Refrigerant circuit-   20 Economizer heat exchanger-   22 Intermediate refrigerant tube-   99 Control unit-   902, 903 Heat source water tubes-   904, 905 Intermediate water tubes-   910 Water circuit-   911 Flow rate ratio adjustment mechanism

CITATION LIST Patent Literature <Patent Literature 1>

Japanese Laid-open Patent Application No. 2007-232263

<Patent Literature 2>

Japanese Laid-open Patent Application No. 2002-106988

1. A refrigeration apparatus which performs heat exchange on a water tube system having a water inlet tube to lead water supplied from an exterior to a water branching point, first branching water tubes and second branching water tubes extending from the water branching point, and a water outlet tube leading out to the exterior from a convergent point where the first branching water tubes and the second branching water tubes converge, active refrigerant being in a supercritical state in at least part of a refrigeration cycle; the refrigeration apparatus comprising: a main expansion mechanism arranged and configured to depressurize the refrigerant; an evaporator connected to the main expansion mechanism, the evaporator being arranged and configured to evaporate refrigerant; a first compression element arranged and configured to draw refrigerant that has passed through the evaporator, and to compress and discharge the refrigerant drawn into the first compression element; a second compression element arranged and configured to draw in the refrigerant discharged from the first compression element and to compress and discharge the refrigerant drawn into the second compression element; a first refrigerant tube arranged and configured to draw the refrigerant discharged from the first compression element into the second compression element; a first heat exchanger arranged and configured to perform heat exchange between the refrigerant passing through the first refrigerant tube and the water flowing through the first branching water tubes; second refrigerant tubes arranged and configured to connect a discharge side of the second compression element and the main expansion mechanism; and a second heat exchanger arranged and configured to such that refrigerant passing through the second refrigerant tubes exchanges heat with the water flowing through the second branching water tubes, and does not exchange heat with the water flowing through the water inlet tube.
 2. The refrigeration apparatus according to claim 1, further comprising: a flow rate ratio adjustment mechanism arranged and configured to adjust a ratio between a quantity of water flowing through the first branching water tubes and a quantity of water flowing through the second branching water tubes.
 3. The refrigeration apparatus according to claim 2, further comprising: a heating capacity detection unit arranged and configured to detect a capacity of the refrigerant passing through the first heat exchanger to heat the water and a capacity of the refrigerant passing through the second heat exchanger to heat the water; and a water distribution quantity control unit arranged and configured to adjust the ratio between the quantity of water flowing through the first branching water tubes and the quantity of water flowing through the second branching water tubes by controlling the flow rate adjustment mechanism in accordance with a ratio between the heating capacities of the first heat exchanger and the second heat exchanger detected by the heating capacity detection unit.
 4. The refrigeration apparatus according to claim 1, wherein the second refrigerant tubes have a third refrigerant tube connecting the second heat exchanger and the main expansion mechanism; and the refrigeration apparatus further comprises: a fourth refrigerant tube arranged and configured to connect the evaporator and an intake side of the first compression element; a third heat exchanger arranged and configured to perform heat exchange between the refrigerant flowing through the third refrigerant tube and the refrigerant flowing through the fourth refrigerant tube; a third heat exchange bypass tube arranged and configured to connect one end and another end of a portion of the third refrigerant tube that passes through the third heat exchanger; and a heat exchanger switching mechanism arranged and configured to switch between a state in which refrigerant flows through the portion of the third refrigerant tube that passes through the third heat exchanger, and a state in which refrigerant flows through the third heat exchange bypass tube.
 5. The refrigeration apparatus according to claim 4, further comprising: temperature sensory units arranged and configured to sense a value of at least an air temperature surrounding the evaporator or a discharged refrigerant temperature of at least the first compression element or the second compression element; and a heat exchange quantity control unit arranged and configured to control the heat exchanger switching mechanism and to increase the quantity of refrigerant flowing through the portion of the third refrigerant tube that passes through the third heat exchanger when the air temperature is higher than a predetermined high-temperature air temperature when a value sensed by the temperature sensory units is an air temperature, or the refrigerant temperature is lower than a predetermined low-temperature refrigerant temperature when the value sensed by the temperature sensory units is a refrigerant temperature.
 6. The refrigeration apparatus according to claim 1, wherein the second refrigerant tubes have a third refrigerant tube connecting the second heat exchanger and the main expansion mechanism; and the refrigeration apparatus further comprises: a branching expansion mechanism arranged and configured to depressurize refrigerant; a fifth refrigerant tube which branches off from the third refrigerant tube and extends to the branching expansion mechanism; sixth refrigerant tubes extending from the branching expansion mechanism to the first refrigerant tube; and a fourth heat exchanger arranged and configured to perform heat exchange between refrigerant flowing through the third refrigerant tube and refrigerant flowing through the sixth refrigerant tubes.
 7. The refrigeration apparatus according to claim 6, further comprising: temperature sensory units arranged and configured to sense a value of at least an air temperature surrounding the evaporator or a discharged refrigerant temperature of at least the first compression element or the second compression element; and a branched quantity control unit arranged and configured to control the branching expansion mechanism and to increase the quantity of refrigerant passing therethrough when the air temperature is lower than a predetermined low-temperature air temperature when the value sensed by the temperature sensory units is an air temperature, or the refrigerant temperature is higher than a predetermined high-temperature refrigerant temperature when the value sensed by the temperature sensory units is a refrigerant temperature.
 8. The refrigeration apparatus according to claim 6, further comprising: a water temperature sensory unit arranged and configured to sense a temperature of water flowing through any position in the water tube system; a first refrigerant temperature sensory unit arranged and configured to sense a temperature of refrigerant passing through the first refrigerant tube; and a refrigerant distribution quantity control unit arranged and configured to control the branching expansion mechanism and to increase the quantity of refrigerant passing therethrough when a difference between the temperature sensed by the water temperature sensory unit and the temperature sensed by the first refrigerant temperature sensory unit is less than a predetermined value.
 9. The refrigeration apparatus according to claim 1, wherein the second refrigerant tubes have a third refrigerant tube connecting the second heat exchanger and the main expansion mechanism; and the refrigeration apparatus further comprises: a branching expansion mechanism arranged and configured to depressurize refrigerant; fourth refrigerant tubes connecting the evaporator and an intake side of the first compression element; a third heat exchanger arranged and configured to perform heat exchange between refrigerant flowing through the third refrigerant tube and refrigerant flowing through the fourth refrigerant tubes; a fifth refrigerant tube which branches off from the third refrigerant tube and extends to the branching expansion mechanism; sixth refrigerant tubes connecting the branching expansion mechanism and the first refrigerant tube; and a fourth heat exchanger arranged and configured to perform heat exchange between refrigerant flowing through the third refrigerant tube and refrigerant flowing through the sixth refrigerant tubes.
 10. The refrigeration apparatus according to claim 9, further comprising: temperature sensory units arranged and configured to sense a value of at least the air temperature surrounding the evaporator or the discharged refrigerant temperature of at least the first compression element or the second compression element; and a branched heat quantity control unit arranged and configured to control the branching expansion mechanism and to increase quantity of refrigerant passing therethrough when the air temperature is lower than a predetermined low-temperature air temperature when the value sensed by the temperature sensory units is an air temperature, or the refrigerant temperature is higher than a predetermined high-temperature refrigerant temperature when the value sensed by the temperature sensory units is a refrigerant temperature.
 11. The refrigeration apparatus according to claim 9, further comprising: a first heat exchange bypass tube connecting one end and another end of a portion of the first refrigerant tube that passes through the first heat exchanger; and a bypass switching mechanism arranged and configured to switch between a state in which refrigerant flows through the portion of the first refrigerant tube that passes through the first heat exchanger, and a state in which refrigerant flows through the first heat exchange bypass tube.
 12. The refrigeration apparatus according to claim 11, further comprising: temperature sensory units arranged and configured to sense a value of at least an air temperature surrounding the evaporator or a discharged refrigerant temperature of at least the first compression element or the second compression element; and a bypass control unit arranged and configured to control the bypass switching mechanism and to increase using the quantity of refrigerant flowing through the portion of the first refrigerant tube that passes through the first heat exchanger when the air temperature is higher than a predetermined high-temperature air temperature when the value sensed by the temperature sensory units is an air temperature, or the refrigerant temperature is lower than a predetermined low-temperature refrigerant temperature when the value sensed by the temperature sensory units is a refrigerant temperature.
 13. The refrigeration apparatus according to claim 9, further comprising: a water temperature sensory unit arranged and configured to sense a temperature of water flowing through any position in the water tube system; a first refrigerant temperature sensory unit arranged and configured to sense a for sensing the temperature of refrigerant passing through the first refrigerant tube; and a water-correspondent refrigerant quantity control unit arranged and configured to cont the branching expansion mechanism and to increase the quantity of refrigerant passing therethrough when a difference between the temperature sensed by the water temperature sensory unit and the temperature sensed by the first refrigerant temperature sensory unit is less than a predetermined value.
 14. The refrigeration apparatus according to claim 1, further comprising: a first drive unit arranged and configured to drive the first compression element; and a second drive unit arranged and configured to drive the second compression element independently of the first compression element.
 15. The refrigeration apparatus according to claim 1, wherein the first compression element and the second compression element have a shared rotating shaft in order to perform compression work by rotatably driving each of the first and second compression elements.
 16. The refrigeration apparatus according to claim 1, wherein the active refrigerant is carbon dioxide. 